Aye, people can be happy to play either role, or solidly set on only one side. You'd have just as many folk who are unwilling to be the dominant party, insisting on always being the one tied-up and done-to. So no, it's not weird in and of itself that he enjoys dominating and doesn't enjoy submitting; it's also not weird that you're comfy with the idea of playing it turn-about. Just individual preferences is all.
However, as you say, her mentioning it could indicate she's not as keen as he. Maybe she was just gossiping about her love life, no hidden agenda, and is quite happy with the set up. But can't hurt to talk to her about it and see if she really is a little concerned.
In the same way that we can't condemn him outright for considering himself "pure dominant", so too can he not be miffed if she turns around and says that that doesn't work for her. Whether it's that she's not into it at all, or that she wants to her her fair shot in, if she keeps her mouth shut, it will all go wrong, regardless of how good his intentions may be. Tell her to talk it out with him now. If their paths are too different, then better they end it rather than wasting time and energy being frustrated at the other person's unwillingness to change. And maybe he'd be happy to at least try things her way, even if it is just to humour her and not a desperate need to submit. But they do need to be on the same page, whatever's going on, and that won't happen unless she talks to him candidly about it.
One thing to be wary of is not to let your opinion of how it "should" be colour her view. Maybe she seemed weird talking about it, not because she's unhappy, but because she knew you'd disapprove? If you want to help her, then you do have to accept that this is something that can work for people, with both sides being happy, and is in no way lesser or more iffy than your switchy way. Try to be as impartial as you can, assure her that you don't judge or suspect the set-up as it is, as long as she's genuinely happy with it. And try to trust her answers, don't look for signals where there are none. If she says it's fine, and you're still not sure, then better you show that you trust her judgement and are happy for her, and let her know you're there, as that way if it is all a bit fishy, she'll still be happy coming to you for help when she's able to: pushing her when she doesn't want to say anything's wrong will just make her feel like you'll be thinking "told you so!", and feel too embarassed to bring it up again until things are REALLY bad.