Ask him. Simple as that. I know it may not be at all simple, especially if you guys' relationship is falling/has fallen apart, but there's no way whatsoever you can avoid awkwardness in the future if you don't talk to him face-to-face now. Tell him you have feelings for this girl and would really like to date her, but only if it wouldn't cause any conflict between the two of you. And be very
aware of how he responds- if he shrugs his shoulders nonchalantly and says, "Whatevever, I don't care" but won't look you in the eye, chances are he cares very, very much. It's a touchy subject that'll require some finesse on your part. And who knows- your consideration in asking him may even help bring your friendship back together.