They would have to just hang out and not really be able to leave for 5-6 hours while our other friend is working, but could do their own thing. Basically they are just there for safety reasons if anything happens to our friend doing the work who we have a fair price established. First friend has a lot of sedentary hobbies so boredom isn't a risk but I want to treat them fairly.
I owe you one, mate (~3-4h equivalent) and a dinner (for pushing into 6)? Or just ask them, if you want monetary relations (or a nice hobby gift)
Our friend group has a pretty good informal time-bank/work-trade situation but the volume and frequency of work in the next period is more than is reasonable to ask and that I can contribute. I'm tapped out with work and other obligations otherwise I would just be the assistant to my friend doing the main work on weekends but working the weekends too has been killing me.