Whoa. That is quite the mess, fo' so'.
I agree with the general idea of the advice being given. Slip in little bits of info in a casual way.
Spread it on thick: you can easily pass this off as motherly concern
Since Joe's sexuality is somewhat unknown, and young people have a way of sneaking things under the radar: I would think this would be a situation to go ahead and be bold. You do not necessarily have to disclose Reggie's HIV status( it may be illegal: I don't know) ; but you could speak about HIV and sex in a general way.
What's the worse that could happen if you did this? You could embarass him. Big deal. In the process of bringing these things up: you will give them both the message that you are paying attention. That you are 'watching'.
It sounds like Reggie doesn't much care what others think/ or how they are affected at this point in his life. If that's the case, it would definetly be a plus to 'let your presence be known'.
BTW; I am not 100% sure on this, but I think that is some places if a person is HIV positive, and they have proven themselves (or you can give good reason) that they are putting others at risk by not disclosing their status, the above legal concerns are nixed. Basically; if someone has good reason to believe that someone who is HIV + is putting others at risk; they should go ahead and speak up without worrying about any legal liabilities. Makes sense to me . Even though I explained it as clear as muddy water
But what do I know? I've never had to deal with this sorta situation