Erm, is he treating her badly? Is she particularly vulnerable for some reason?
If so, talk to her about it, or if you don't think she'll listen to her, then perhaps speak to her parents, or someone else who can support her.
If not, leave her alone and stop being prejudiced. I met my boyfriend when I was 17 and he was 29, we are ideally matched and still very happily together four years later. Age isn't an issue.