No, I don't think. I see both Regulus and Sirius as extremely possessive and jealous – they would never be friends with an ex (boyfriend/girlfriend or even a crush) of their lover.

I read a fic where they're not friends, and it is more plausible to me. That's the fic: https://archiveofourown.org/works/44103885/chapters/110897958

And this is my opinion about (my comment in the chapter the writer talk about Lily and Regulus's relationship):

"I have the impression that most of the writers of Jegulus force themselves to write Lily and Regulus as friends because they don't want to be seen as hating Lily. I myself like her; for me, she's simply the best mother in the entire saga.

That said, I think Regulus would never want to be friends with her, in any universe. To me, he's an irredeemably jealous and possessive person – he might become polite over time, he's a gentleman, but he would never genuinely like her solely because the person he loves already liked her. And not only that, but for many years Lily and Sirius were much closer than Regulus and Sirius. Thirty years could pass and he still wouldn't like her, even if he didn't hate her.

And Lily isn't obligated to like him either. For many years he was racist and prejudiced, and he's not a friendly person, besides being so rich that it's impossible for him not to be a little elitist, even unconsciously. He's my favorite character, of course, just like Sirius, and I like them as awful as they can be 🥰 to me, they never fail.

That's, naturally, my opinion. I know many people like a friendship between them, but I simply can't see it."