Is Such a Union Allowed in Islam?

As-salamu alaykum everyone. So I've got this friend who follows Buddhism, and she's been seeing this guy who's Muslim-he's half Sudanese, half Ethiopian. Right from the start, I pointed out that, given Islam's teachings in the Quran and his active practice, he might not be serious about her since she isn't Muslim. But she won't hear any of it; she's convinced it's a genuine, lasting bond. I'm not Muslim myself, but I have Muslim pals who've shared that they doubt his intentions, noting that a Muslim can't actually marry someone practicing Buddhism. They've been together for over a year now, so she's probably really holding onto hope that it'll end in marriage. Maybe I'm off base, but she just isn't listening at all.