That is just my opinion some people chose to smoke some chose not to that is just my advice if you would like to help him quit smoking but ultimately it is his choice to do so.
Yeah then you should ask him to quit smoking. The first step to keep his immune system healthy and that starts by living a healthier lifestyle. If he does want to quit but has a hard time doing so tell him to ask his doctor if they having smoking cessation which is to help people to quit smoking. He can also go cold turkey but if he does make sure he is doing some active and be there for him because having a good friend around to make sure you don't slide back and light up is really helpful.
My friend has HIV and smokes realy bad.
Why are you worried if someone with HIV can smoke do you have HIV. If not would not concern myself with it to much and by the way if you do smoke you should try quitting and live a healthier life style but that is just my opinion