Britain's Dujardin, a six-time Olympic dressage medallist, withdrew from the Paris Games on Tuesday after the video emerged, saying it showed her "making an error of judgement".
The video, obtained by the BBC, shows Dujardin repeatedly whipping a horse around its legs during the session.
After her withdrawal from the Olympics, the 39-year-old was provisionally suspended by equestrian's governing body the FEI, which received the footage on Monday.
Dujardin said in a statement: "What happened was completely out of character and does not reflect how I train my horses or coach my pupils, however there is no excuse.
"I am deeply ashamed and should have set a better example in that moment."
Speaking to BBC Sport, the whistleblower's lawyer, Stephan Wensing, said his client had mixed feelings about the reaction since the news broke, but she believed it is a widespread issue in dressage.
"It's not fun to ruin a career. She's not celebrating; she doesn't feel like a hero," he said.
"But she told me this morning this had to be done because she wants to save dressage."
On Wednesday Dujardin had her UK Sport funding suspended pending the outcome of the FEI investigation, while she has also been dropped as an ambassador for horse welfare charity Brooke, which said it was "deeply disturbed" by the video.
"Our whole ethos is around kindness and compassion to horses, and to see the opposite of this from someone with such a high profile is beyond disappointing," it said.
Two of Dujardin's sponsors, equestrian insurance company KBIS and Danish equestrian equipment company Equine LTS, have removed their backing.
Equite LTS said they are "shocked and saddened by the video" and "do not condone this form of behaviour".
KBIS said they "cannot and will not condone behaviour" that goes against providing the "best care possible" for horses.
Dujardin had been set to compete in both the individual dressage and team event alongside Carl Hester and world champion Lottie Fry, on horse Imhotep.
She needed a medal of any colour to take the outright lead as most-decorated British female Olympian from now-retired cyclist Dame Laura Kenny.
On Tuesday Dujardin said the video was "filmed four years ago", but Wensing said it was from two and a half years ago.
"When she filmed this and was aware of this two and a half years ago, she was thinking everything this superstar, the best rider, is doing, must be OK. This must be the way to train horses and how to deal with it," he said.
"Charlotte Dujardin was explaining during the lesson that she wanted the horse lifting the legs up more in canter.
"Later on, [the whistleblower] was thinking 'this is not OK'. She had spoken with several people in the profession and they all warned her 'don’t fight'.
"She was really afraid. There was a sort of fear culture and she was also thinking 'when I do something, it will be victim-blaming'."
The timing of the release of the video has also been questioned, with Madeleine Hill, a former dressage reporter for magazine Horse & Hound, telling the BBC Radio 4 Today programme it felt like "sabotage".
She believes the FEI should have waited until after the Olympics before imposing any sanctions, adding that "top riders are being persecuted" on social media by people who are against the use of horses in sport.
However, Wensing said it was the recent removal of a rider from the Denmark dressage team that encouraged his client to report Dujardin.
Earlier this month Danish TV station TV2 reported, external that Denmark's reserve rider Carina Cassoe Kruth had been replaced in the Paris squad on the eve of the team announcement after a controversial training video was sent to the Danish Riding Association.
Kruth told TV2 she "deeply regretted" her "clear error".
"Because of the Olympics, [the whistleblower] was thinking if I don't do anything now [Dujardin] will probably win medals," Wensing said.
"On the other hand, people are thinking wrong that she could have done this during the Olympics, and that would destroy the whole British team.
"Now the team can organise themselves and use the alternate. It's not like the whole British dressage team has gone now. There could be a worse timing."
I guess my question is; Is that excessive? Or is that just how you get a horse to run weird? Did this lady take the L because the broader implications?
Yeah, this is really not normal. I grew up on a farm and when I heard about this on the news I kind of rolled my eyes - people can be freaked out by how you interact with horses because it can look a little rough, but they're massive fucking animals with stone/metal fists and will kill you if you actually hurt them. Almost always, a horse is fine. And then I saw the footage.
This is really not the way to do things. What she's doing is both excessive and incredibly unprofessional. Straight up amateur shit. In my village the tradition was that you don't give a kid a whip until a) they're 13 and b) they have proven that they can ride without it. You never need them if you know what you're doing. Whips/crops are supposed to be used to surprise the animal and get their attention, not to hurt them. The type of whip she's using in this video (a lunge whip) shouldn't really even make contact with the horse, you use them to either make noise behind them or to shock them a little from the corner of their vision.
The insane part is that at one point, she's hitting the horse from the front. That is something you never fucking do (partly because you're increasing your chance of getting fucking killed lol)
Like, this is something that you might expect of someone who has never handled a horse before and is particularly mean. This would never be called for under literally any circumstances. To see this from someone who's been riding their whole life and absolutely knows better? She was pissed off and actively taking her frustration out on that poor horse. I doubt she could really do much damage with a lunge whip, but she meant to hurt the horse.
Firing squad for this freak immediately
EDIT: i've just watched it all the way through and she's using two fucking hands??? also at one point i think she's literally impacting the poor thing with the handle of the whip at some point??? she's doing so many things that are unnessecarily and unprofessionally cruel and also could get you fucking killed. like, surprisingly enough, forcing a massive animal up agaisnt a wall is a very very bad idea. it almost kicks her and she still doesn't back off. evil shit.
that looks like it might be fairly routine the woman is very casual and doesn't snap the whip every time, the comment that the whip isn't hitting hard enough probably took it over the line and the sport has to save face. i'd definitely put money on this being accepted practice