i am so glad you asked okay i live off of the byler secret relationship cocaine. outside of the party i want to say joyce, but i can't. even before the great nuke of s5. she was so busy all the time and although she knows mike and will so well & loves them dearly, i think jonathan is most likely to pick up on a shift in dynamic — especially with will. jonathan can read his brother SO well, we saw this in s2 & s4. once mike and will start spending a questionable amount of time together (which is what they always did as kids), he'll feel a bit of sympathy for will, thinking they're still only friends until he picks up on the aforementioned shift. idk. i think he'd see it in will first, then mike, then it would all make sense.

within the party i think it would be lucas. he's really attentive and has known mike and will for way longer than max has. dustin is obviously another candidate for this, but i feel that he's always so busy outside of the party doing a bunch of different things that he'd be too distracted to notice. lucas would probably notice something on mike's side first tbh and get all nosy. but mike is defensive so he turns to will and then will gets a little cagey too so he's like. wtf. and just keeps an eye out for things whenever they hang out. until it all makes sense. and lucas is a little awkward about trying to bring it up but he's a sweetheart and he's trying his best