Hi, first of all, congratulations on your own weight loss! That's great for you but is probably making your boyfriend insecure even though he wants to be happy for you.
The best way, I think, is to sit down with him and tell him what you need him to do to help YOU maintain your weight loss.
For example, you need him to exercise with you--run, walk, go to the gym, ride bikes, generally be more active. Tell him that you need him to make healthy food choices when you go out to eat to help you not be tempted. When cooking at home, you need the two of you to try healthy, low-calorie recipes to help you maintain YOUR weight loss. If he gets on board, he'll get in better shape too.
The best way for him to lose weight is to eat less and exercise more--it's as easy as that! The two of you could go on a special diet together one week out of the month, as maintenance for you--but in reality, as a quick way for him to jump-start his weight loss and become motivated.
PS--He knows he needs to lose weight! He just may not care right now. Anything you say is going to make him feel more insecure. Go through the "back door" and make it all about maintaining YOUR weight loss.
You're probably the only one that could say something to her and get away with it. She knows you care about her, so approach it that way.
Tell her that you're worried about her health and you want to start taking care of your own health. Explain to her how difficult that is when you're with her eating excessively.
See if you can talk her into going on a "health kick" with you where you learn about nutrition together, exercise together, and get healthy together. You can act like YOU need it (maybe not the weight part, but the getting healthy part.) If you could figure out a way to get her on board--that would be the greatest gift you could give your friend. Good luck!