Joint hyper mobility and strength training

[rocketcity]

If someone has hypermoblity in their joints is it still safe to do strength training?

I'm asking because my roommate has extremely flexible joints where his arms and knees naturally hyper extend when he locks them in place. We trained together a little about a year ago before I learned about strength training but he over extended his knee running and stopped. I went on to do strength training and he went to a no carb diet and lost over 80 lbs. I noticed his diet has stalled and when we worked out again recently his strength had gone down significantly in all the exercises we did before he went on his diet. I think building up some muscle would help his metabolism a lot because it made a huge difference for me but i'm not sure how to train him because of his joints. Squatting with him his scary because he had decent leg strength for novice (185ish lb)but sometimes would mess up and over extend his knees putting a lot of pressure there. Bench is the same with his elbows. I'm worried he will hurts his knees, less worried about the elbows in bench because he can see when he overextends. I was wondering if you have any experience with new lifters like this and if there are any methods to help him prevent overextending his joints on accident?

[Mark Rippetoe]

Your roommate can train just like lots of hypermobile females do. He learns the correct position for knee and elbow lockout, and that's where he stops.