Hi Ali! First of all, I have too many characters, so if anyone I didn't mention is useful to you, just let me know! That said, some possibilities are...

Jay Fox lives and works in London (Knockturn Alley) and is involved in ~slightly shady business. As such, he also largely keeps himself to himself, but otherwise he's quite sweet and lonely and would be a loyal friend if he grew to trust her.

Titus Cripps is also working class and close in age, and unlike Jay is the most trusting person on the planet. He had a few years at Hogwarts but now works at the Sanditon beach resort for a ghost gentleman. Still, he can easily venture out into the wider world, and he'll make friends with anyone! He's usually better at making friends with girls to begin with because he finds them less intimidating. Depends how much patience Briar has for a goof!

Jude Wright is my one of the Revs (like Leeny above!) and they mostly hang out at the Augurey Beak Cafe so if she wandered in there she might get to know any of them! They are usually campaigning for workers' rights and absolutely support blood equality, squibs included.