A good friend is not someone that will just tell you what you want to hear

MayCaesar

I think it is important not to force your views on your friends. If your friend is overweight, but does not want to talk about it, then pushing the topic in order to try to get the friend to do something about it is a bad idea.

Also, if your friend seeks some validation and wants you to tell him/her that being overweight is okay, then, depending on the situation, you might want to oblige - not to lie about it, but to be warm and diplomatic, so to speak. You could say, "You may be overweight, but I still have got your back. It does not change my opinion on you in any way".

However, a truly good friend manages to inspire his/her friends to change in a positive manner, in a way that makes them actually want to do it. It is not easy to do so, but if you act from empathy and not from insecurity or some other negative aspect of you, then you are likely to succeed.

I got one Korean girl to stop smoking by having her back at hard times. She was having some psychological issues due to some events that happened to her, and smoking extensively was a way for her to cope with those issues. By being there for her and serving as a better outlet for her feelings, by supporting her and having chats with her, where she would unload all of her feelings on me and I would take it all in and give her encouragement, I got her to confront her feelings, instead of hiding from them. And simultaneously I gently promoted healthy lifestyle, offering her to go for runs together (which she did not accept, but she appreciated the gesture), going for walks together and so on. I only knew her for a few weeks (I was in Canada for a month), but by the end she quit smoking altogether.
I also got one French girl to drop a lot of weight, to the point where I could hardly recognise her when I met her again a year later. But she, to be fair, wanted to drop weight desperately, and was ready to have difficult discussions about her weight issues. It is always easier when the person actually wants to have these conversations, than when you have to gently steer them towards them.

Human relationships are sophisticated things, and there are no universal rules applicable to every relationship. A good rule of thumb is to never lie and to always be authentic, but there is a lot of nuance pertinent to individual cases. I usually prefer to get somewhat close to the person before initiating hard discussions, and I need to know what they are comfortable talking about before I choose the "plan of attack".

Also, humor is to not be underrated. Humor breaks barriers and melts people's hearts. And when someone seems especially serious and resistant to humor, typically it means that humor is what they need the most right now. If you can go through the initial resistance and employ healthy humor despite their seeming unwillingness to listen to it, you can often get them to open up to you and tell you of deep issues you never suspected were there.
But it should not be done in a manipulative manner. You have to be genuine in your intent, otherwise the manipulation will be detected and heavily rejected.

smoothie

If my friends told me everything I wanted to hear all the time then they would be yesmen not friends

I like hearing opinions about me from people close to me because its important to learn another's perspective on your own character. Nobody is perfect and its good to hear that about you sometimes