I feel like the MMA guy was in the wrong in this situation.
I currently am a brown belt in BJJ but I go to a nearby high school every single day to wrestle in order to make sure my wrestling also doesn't stop improving.
Although a lot of what is being taught may not be directly applicable to BJJ or MMA, I understand what I am being provided, which is a FAVOR from the team/coach. Before finding the current school where I am wrestling at, I've been turned down from another one because they don't want strangers wrestling with the team.
If the coach says to do a double leg, I'll do a double leg, but if he also says to practice cradles and tilts, I will do it as well. I'll sit out if it means none of the kids on the team have to sit out because of a lack of a partner, and I will try my best to not cause any injuries.