Have you ever had doubts about your new BF before your ex intervened? Don't worry about it and don't it let it consume your mind, which can potentially ruin the relationship in the long run. You heard this from your ex, so there is may be motive behind the reason why he decided to tell you about this. It's rather obvious, so think about it.
If you're too worried, ask your new bf if he was ever interested in your friend. If he doesn't want to respond or responds differently, that's good news. But even if he did say yes, don't worry about it. Wanna know why? Because he asked you to be his gf and you have him, not her. If he's playing you, then you would quickly find out. Talk to your friend about this and see what she says- she may even help you out with this, or if anything, you're telling her what's up and to keep an eye on him.
Always remember this though. The most important part in every relationship is communication. Talk about things, especially if it bothers you. But also tell them how much they mean to you. It works both ways and if one is missing, it can really damage a relationship further down the road. so be truthful and don't try to play games with him, either.