Your friend should not go the concert, not under those circumstances. Even if the guy did tell her to go with him again, wouldn't it be awkward? I mean, wouldn't she be going knowing that he had asked someone else to go with him? That would defeat the whole purpose of it, don't ya think?
Well, if he physically has the tickets and already offered one to a different girl, she can't technically go anyway forcing the other girl to purchase her own ticket.
Honestly, the whole situation sounds silly. Why commemorate an anniversary if you're no longer dating?
If she bought the ticket, then I'd say yeah, she should go. But if not? Well, it's up to her I guess. The whole concert could end up being a really awkward, angry experience if she's upset with him and he's mad at her.