They laughed. Apparently picking up Spanish from your Spaniard friend has some downsides.
As in South American backpackers find it funny/amusing this drunk Chinese guy in an Australian pub used the Castellano equivalent of "y'all" in a sentence.
They were ultimately nice about it, much better than the time my (English speaking) Canadian coworker tried to speak to some Parisians in school level Québécois and they scoffed and continued in English.
I'm not a Espanyophone so take my opinion with a grain of salt... ustedes just seems normal, for both standard and slang use...
Oh I thought you meant "standard" as in "formal", my bad. Yeah I'm not Spanish either so ustedes is what's i'm used to too