Gosh, good question. The answers are great. Lead by example. Just keep doing what you are doing, minimal conversation regarding your concerns. I suspect that your friend knows the situation, just be supportive and live your life the way that you need to. She/he knows the situation and will, hopefully, come back.
I agree that it should be handled as delicate as possible as to not hurt his/her feelings. I would keep the lines of communication open maybe open a conversation about getting healthy but not necessarily directed towards them. This could lead to them ultimately asking for help or advise and opening the door for you to step in.
Truthfully- I stress negative reinforcement sometimes. It may be bad in the short run, but if you really want something to stick in someones mind dont be afraid to tread on their feelings.
Some of my best clients I grabbed a handle on their waist and said “we’re going to get rid of this”. I wasn’t insulting, but I was pointing out.