First things first - don't believe what he's said about what her bf thinks. a) why would you care what he thinks? Your feelings and opinions are yours, and you don't have to change them if someone has a different view. b) your bf is very likely making up what the bf thinks, to try to convince you that you're being unreasonable.
Yes men and women can be platonic friends. It would be unreasonable to expect him to drop female friends.
Do you trust him? You think the 'OW' fancies your bf, and it comes across that you think that, given the opportunity, she would be in a relationship with him. If you trust your bf completely, it would be imaterial if she fancies him - you'd know he'd do nothing about it.
However, I can see why you might not trust him. He appears to be spending time with a women he knows fancies him, arranged to stay over at her house, and when his gf is concerned, tries to convince her she's being unreasonable / imagining things.
Something I learned when I was a few years older than you are now:- when a man really likes you, you know it. No questions, no doubts, he doesn't mess you around or lie or give you mixed messages. He doesn't go hot and cold. He wants to be with you, loves being with you. It doesn't mean he drops everyone else, but you know you're important to him.
It might just be you aren't right for each other. It could be that there is something going on with the other woman. It could be that he secretly enjoys her attention; his ego likes the idea that 2 women want him. He certainly doesn't prioritise making you feel better about the situation.
I think I'd chalk this one up to experience and walk away. Don't get dragged into discussions about her and him. Just say it isn't working for you and he isn't providing what you want.