If she actually hated him she wouldn’t care enough to like the posts that he was tweeting about unless it’s to get on his nerves. So your boy could be lying. Talk to your boyfriend and let him know it’s bothering you. There’s a line between friendships and something more. If it’s his “best friend” don’t you think you would’ve met her by now? If you can’t tell the difference between how he’s treating you and how he’s treating her I’d say ditch the guy.