Giving my best friend bf advice?

She says she loves him and likes him a lot, but I'm just not really all for their relationship. He's a nice guy but there's an awkward age distance, some weird vibes and a few other things that put me off towards her looking for a long term relationship with him. Lately she's been asking for advice since they got in an emotional fight and has questioned about staying with him... Would it be wrong to tell her I think she should break up with him just because I don't like him? It's her first relationship too so I feel guilty for wanting her to end it with the guy.

Edit: I have been withholding these feelings from her and been supportive of her up to this point. Their fight has left her recognizing some potential "flags" that slightly bothers her, hence her seeking support/advice.