I sure this has already been said, but I’d follow these three steps
1) Educate yourself on the subject (this is important regardless of his status). Not saying that you’re not already, but it’s always good to try and learn as much as possible.
2) Do not confront them on the matter or discuss this with other people, particularly those they are connected with.
3 Demonstrate that you are not prejudice against the those living with HIV and AIDS. At least then they know they know you are an ally, even if they never discuss it with you. An effective and natural way of doing this would be criticising the way that governments are defunding education and prevention programmes. Much easier to work into a conversation than you would think, especially if you’re into politics.