This is an extraordinary tricky situation. Obviously, something is going on that prompted your friend to gain all that weight. I have no doubt that he/she is painfully aware of the extra weight and its possible consequences for the health. Many people are not very diplomatic on making their opinion known about it.
He/she has not asked you for advice, I assume, yet is aware that you are in the fitness industry and may be in a position to give assistance.
What I would do is keeping the channels of communication open, not saying a word about ‘healthier life choices’, all the while remaining a good friend and be there.