Anywho, I apologise for slacking on your intro post. I'm a fail boat, but hopefully what I have to say will be beneficial and/or helpful! Violet, Toby, and Sam are in his year, so would at least know of him.
The strongest link would be Violet, seeing as she's in both his year and house. She's lovely (as I'm sure I've said about a million times already) so she'd definitely be friends with Jack! Both before his transformation and after it. I completely offer her up to be close friends with Jack!
Sam would more than likely have been his friend pre-makeover, on account that he isn't especially popular himself. It's totally up to you whether or not they're still friends, but Sam would only sever ties should Jack begin to act differently. So that ball is in your court. They're both Prefects as well, so they share that in common! Yay! I totally suggest friends, even though Sam is a little anti-social and quiet.
As for Toby, he's a pretty friendly bloke. I offer him up to be a casual mate as well! He's a bit of a nutter but enjoys having fun and is a nice guy to know. I hope these two can get along!
Unfortunately, all of my seventh years would have no idea who he was! That sucks, but if you ever want random interaction, I have three of them for you to pick from!