Marauders fans I have an idea
Imagine James Potter noticing a pattern in Remus behavior : missing class once a month, spending some days in the infiarmery, being moody and particularly tired, eating a ton of chocolate...
All of that remind James of his Mom and so he though he discovered the secret of his friend.
Remus is trans, that's make sense, he probably don't want his classmates to know so unlike the girls he has access to the infiermary, plus he must be in huge discomfort because boys don't have their periods and Remus is a boy but he has periods. Keep in my mind that James is pretty young at this time, he knows about trans people thanks to his parents, but also knows that the wizarding world is not as open. He also doesn't know who he could trust, so he doesn't say anything nor to Sirius nor to Peter.
When the time comes once a month James is particularly careful with Remus, offering him chocolate, asking for the boys to be calm in the dorm the night before and the night when Remus come back, pretexting that him, the James Potter is tired, taking good notes in class for the missing boy ( even though Evans will lend him her notes )...
Remu notice of course, he doesn't know what to think, James isn't supposed to know about is wereabouts, so as much as he's grateful for his attention he's also scared. How James could have known in a few months, and still not say anything.
Lily Evans, the smartest witch of their generation, is having the time of her life watching James. How can he be so smart yet so wrong ? How this young boy coming from one of the oldest wizard family could have think of something so muggle ? She's eleven and watch Potter being the best of the friends, not telling anything to anyone, while doing a lot of thing to help Remu. He doesn't explain why, he tells no explanation and yet the other accept the routine and eventually follow his lead, sometimes it's time for jokes and sometimes for calm, it's a welcome break for every Gryffondor first year . Lily watch and find this cute and generous how he is that good of a friend for something he have absolutly no clues about.
Watching the interanl turmoil in James is very fun, until it isn't. Remus is more and more scared by his friend behavior, he's paranoid, what does James want, has he told anyone, what is he waiting for.
Lily seeing that James doesn't know how or what to ask to Remus in order to respect his privacy while Remus fear grows more and more decide to step in.
On one their afternoon tea she tell Remus, she knows he's a werewolf, she has knew for a very long time. James on the other hand misread the clues. She didn't ask for him to tell James about the truth, but he did.
Remus John Lupin is a Werewolf, and James Potter and Lily Evans are still his best friend.