If he's in a relationship and that's his reason, he should say that. So he may be trying to keep makare's friend on the side, in case the relationship doesn't work out, without letting her know that's what's going on.Seems more like some teen-drama TV show bullshit.
If friend has told Kurt she's interested, and he has any reason for not trying get into a relationship with her, it's because he doesn't really want one. If she hasn't told him, then she'd better, and if she doesn't, it's open season. Now, if he's in a serious relationship with someone else, and that's his reason for not being with her, then, well, them's the breaks. Sometimes the one you want to be with is better off with someone else.
One way or another, he sounds like a tool.