I was in the SAME situation when my ex-roomate had Gus the vieled cham.
He was a nice person, he was just neglecting Gus and telling him the proper way to care for him got me no where. At the same time, I couldn't freak out because this person lived here and was a personal friend at the time.
What I did was fall into a medium somewhat. Instead of regular gifts, for Christmas I bought Daryl some UV lights to use on Gus. Birthday, same thing...a misting bottle and some white heat bulbs.when I'd go to the pet store or the show I'd pick Gus up silkies and mealworms etc..he ended up going back to his bad care ways but at least I made these things an option for him to use and also kept our friendship. Later on as you know I got Gus, so it all kinda worked out without major drama aside from Gus getting the crap end of the stick.