My two cents: It's difficult to really know the situation or the nature of the relationship. In general terms, though, I'd say that teenagers in particular often experience spells of conflict or react emotionally to disagreements etc., boundary push and find complex resistances from parents that find the growing up bit hard too!
In a 'normal' case, I'd always promote the maintenance of positive relationships over letting things that might actually be minor fester. In said 'normal' case, I'd always say fostering positive relationships can only be a good thing, and these things often pass in time - but obviously it depends on the issues and whatever is going on, plus the behaviours etc. involved. If he's just being a jerk, clearly it might be different, and she can make her own choices - but things can escalate into real emotional trauma from all sides if alowed to develop. I know I'd be hurt as hell if my child decided not to see me, particularly if there was no reason that was ever broached. Fifteen years in, presumably they've both invested to this point? I'd try to respect my child's view, I guess, but we all now that even 15 year olds often know a lot less than they think they do and can act rashly. I can only speak based on my experience of my ex - but honestly, it would cross my mind that they might somehow have been influenced. Worth considering his feelings, even if that's hard. As such, I would certainly tread carefully, but I'd see it as a parental responsibility to at least try to discuss with both to find out what might be the issue, as in the end it's arguably best for your girl in the long run if it can be sorted. As I say, if the dad just isn't bothering, maybe leave it be until he acts, but sometimes, even if it WOULD be his fault, it's actually in everyone's interest to try to fix things before they stick. Both might be capable of sulking out disengaging out of hurt, and that can become a wedge that eventually creates a huge distance which can be hard to bridge later.
Without knowing more, I really wouldn't want to say - but it sounds like perhaps something that could be solved if they discussed it, or the rootof the problem was found. Guess you could consider these things and just do whatever you think is best for her.