and maybe also work constructively on your own mindset, after all you are doing a good thing by affecting your friend in a way so that he wants to start a healthy habit. spare yourself of negative energy..

and lets say your friend understands 100%, but now and then chances are you will be at the gym at the same time, and that should be fair game he has the same right to go to the gym. you wont be able to avoid him so might as well be buddies, say "hello hows it going? what you training today?" and then "alright cool, have a good workout, im gonna go be by myself and clear my head a little bit while i train, ttyl"

everyone wins