The ones close to them know (like Mouse, Warren, Thorn, Josiah, Rainer, and obviously Dutch and Micah). Most of them don't really care / see an issue with it.. like I think most of them are happy that Jenny has someone she's genuinely close to and know that Micah isn't the best partner for her.

Also before I get into this, ftr, Jenny does not want to be with-with Henry or fuck him, but she does get a lot of comfort from him, they're courting and I'd say she has these little spurs of crushing back at times; they do cuddle like platonic friends and eventually Henry kisses her. She offers herself to him afterwards while in a low state, but he knows she doesn't genuinely want it, and declines, which she later is thankful for, as she's terrified them actually doing anything together would ruin what special friendship dynamic they do have.
Either by changing their dynamic in the way Henry treats her and forever ruining it or him fucking her causing her to view him differently "like all the other guys" in hindsight in a sense. So I'm not gonna say Jenny ISN'T cheating in subtle ways.. they're closer than Micah would like them to be, she's more open towards him while hiding things from Micah, they touch and all, but she also doesn't see it as chasing a different committed relationship. She doesn't want to leave Micah for him / would still ultimately choose Micah.

Anyways,

Rainer wishes they'd run away together and adopt him, he's very pro-BuckLocke, and although he actually doesn't dislike Micah (he respects him a lot, actually), he's aware that they'd be better separated and that the things Micah puts Jenny through are still really unfair and often cruel.

Mouse is probably the guy who'd have the biggest issue with it, because he's Micah's biggest fan and is very strictly like... "You belong to that guy, why would you be sneaking around like this behind his back? You had his kids, do you not think it's disrespectful??" Funnily, Mouse is aroace and has 0 interest in being in a relationship himself so the fact he's invested enough to care is funny, but he still holds these kinds of misogynistic and very anti-NTR views (like even when NTR seems more reasonable.. I'm very anti cheating myself, but I don't blame someone who's in an abusive and neglectful relationship that they literally cannot run away from for finding comfort in someone else either yaknow); and overall, Mouse is suspicious of Henry and her as much as Micah is.. buuuut after he grows to really respect Jenny more, I don't think he'd snitch tbh. He'd probably still have an issue with it, but he wouldn't want to get her hurt.

Thorn and Josiah don't get into relationship business, other than Josiah letting Jenny use him for a short period and not snitching about it pffft. I do not see a reason for him to care after that, and Thorns just generally like "I don't care what any of you people do, as long as you keep me out of it".

Warren is probably more pro-BuckLocke leaning tbh. He's closer with Henry, and knows Henry likes her so he's the type to tease about them being together even if he knows they're not (like that annoying thing your parents do any time you hang out with someone of the opposite sex). It's mostly to embarrass Henry, but doing so is a bit of a dense, dangerous thing to joke about too openly because it could probably get Henry killed or punished if someone like Micah or one of his other nameless goons who don't care for Jenny overheard them and took it seriously.

Dutch doesn't like their dynamic only because of the whole.. "him using Jenny to feel some level of power over Micah" thing, and if someone else gets to have her, then it lessens that blow / the appeal or the point. Like being able to fuck Jenny is better to him because it's something only he can do very openly, knowing it bothers Micah but that he's not going to say anything to him about it, not because he genuinely likes her or is jealous, if that makes sense... not that Henry and Jenny ever fuck or anything, but there's reasonable suspicion and some aspects of their closeness or the idea it could lead to that would still bug him lol. He does catch them kissing, but he doesn't say anything, because his annoyance towards it is kept entirely to himself. He doesn't exactly care about other people's relationship dramas at the same time either.. I cannot ever see him gossiping / snitching about this lmaoo
Dutch also just doesn't like Henry, because Henry has snapped at him a few times and knows that Dutch uses her often and he openly doesn't like and is willing to criticize him for it.

Aaaand ofc Micah hates that fuckin guy. When he's being petty, he'll refer to Henry as her boyfriend in a sort of like.. guilt-trippy, manipulative way because it makes her really mad that Micah doesn't trust her or when he tries to insinuate she's doing things that she's genuinely not actually doing. He wont actually punish Henry because he has no proof they've ever done anything (although first chance he gets, he definitely would.. like I guess if you wanna count the whole IV thing ?), but he is overly suspicious and looking for any kind of confirmation towards his bias. He even stalks them a little bit, standing around the medical den without them knowing so he can listen in on their conversations. It helps him get a little more insight on what's going on when he's not in camp (since she tells Henry practically everything, but only tells Micah what she thinks is important for him to know, especially hiding small issues or mistakes that were already handled if she thinks he'll overreact anyway), and it helps Micah figure out where she is mentally, if it's time to love-bomb because she might be on the verge of snapping and running off again, etc etc.