I think it's wrong for my friend to date his ex's bestie
Why do you think it's wrong to date her?
He says he loves her, but that's because she looks like his ex
Dating someone's ex bestie can be tricky. That his main motivation is that she looks like his ex makes it even less kosher.
He still can't get over his ex.
That's probably the root of the issue. Tell your friend he's wrong for dating that girl with those reasons.