Hey, NWFL ~
This is a very important question, in our current, difficult, world. And - it is really scary, out there.
If a couple, like your friends, are serodiscordant (as per the new vernacular): there are two cornerstones of prevention:
1.) If your friend's HIV-positive lover is being treated for HIV with antiretroviral drugs, and has a low to undetectable viral load: most authorities suggest that this will be a great agent of prevention. However, it doesn't matter if you friend's lover is
feeling healthy or not; he must be tested, take his medication, and then be
tested again, to ensure his viral load is low.
2.) There have been studies that suggest that sero-discordant lovers who use condoms effectively can do a lot to prevent infection, IF (and only IF) they use them constantly, and properly. (And the last part is a big deal, because often people don't use them properly.)
http://www.catie.ca/en/pif/spring-2013/condoms-tried-tested-and-true
https://cdn.metricmarketing.ca/www.nccid.ca/files/serodiscordant_couples.pdf
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NWFL, I think the best news in all of this, is that your friend's b/f has been HONEST with him, about his HIV-status. That's a very good sign, and hopefully they can work together, on this. They undoubtedly need to have some very serious talks about how to cope with this obstacle: but, if your friend's lover is being treated, and is willing to use condoms regularly, my hope is that they will be OK.
I would just say three things, from my own perspective: #1: I am totally paranoid about stuff like this. . . but I would not turn down Mr. K., if he turned out to be HIV positive. (He has not, so far.) But bad stuff happens, in life, and sometimes you meet someone in life who has had one of those accidents in life, but who is still the most lovable person you know. . . and then you have to COPE.
#2: I think your friend and his lover, and their doctor(s) should have a serious chat about this, and figure out the best prevention-strategy, for everybody.
#3: I think your friend's lover should work hard to ensure that he is doing all he can to prevent your friend being infected with HIV, including taking antiretrovirals AND using condoms (especially if your friend's lover is the "top") ~ because that's important.
A lot of guys are living happy and healthy lives in serodiscordant relationships, these days. But it takes a lot of work, caring, knowledge, and patience, on BOTH partners' parts. You have to be vigilant and conscientious about it, that's for sure.
Last, I'd like to wish your friend and HIS boyfriend, a lot of joy and happiness, together. (From what you've told, the b/f sounds very nice - and let's not forget: life kind of sucks, in certain non-good, non-gay, ways. People DO get illnesses, despite being good, and careful, and all that. *I have a weird illness called 'sarcoidosis', which is super-rare, and no one knows how or WHY I got it - but my b/f has to put up with THAT. I could drop dead at any moment, and my b/f has to cope with that - but he still loves me;-)))
So, best to you and your friends, NWFL - I'll pray for them!
Kisses,
"A" XOXOXOXOXOXO
https://www.youtube.com/watch?v=PP9jHD_dRFI