Nobody's actually asking what you use the camera for, so there's no way to tell whether or not the upgrade will benefit you.

I recently bought a T3i to be a second body along with a 7D; buying a second 7D seemed overkill for what I was using it for. Whether upgrading to the 7D makes sense depends on what you're doing with it.

How often do you hit a situation where you can't get the shot you want because your gear didn't cooperate? Are you shooting situations where a high, sustained burst rate would help get the shot? (like motor sports, birds, running animals, etc?). Are you shooting things where improved Autofocus will help you get shots your missing? Are you running into low-light situations where shots are ruined by high ISO noise?

If neither of those is true, then upgrading to the 7D probably won't help you a lot. The t2i does a nice job and turns out a quality image in most situations, and in those cases, I'd be wary spending money on an upgraded body. You definitely would benefit from buying upgraded glass, and that might be a better investment.

From a pure money standpoint, though, if you can get the 7D for half list and it's in good shape, that's a really good deal for a used body like that -- and it might make sense to do and sell the T2i, knowing that in a few years, you can probably sell the 7D again and get a good deal off your friend's willingness to part at a good price.

My guess is that since you aren't explaining technical reasons why the 7D would improve your photography, it probably won't. The image improvement between a T2i and a 7D is noticable but not huge, and you'd see more of an improvement upgrading your glass. Unless you need/want a feature the 7D has that the entry level body doesn't, it's hard to justify doing the upgrade -- and IMHO, the big changes between the bodies are burst rate and the ability to sustain it, better AF, and better high ISO response. If any of those are significant in your shooting, then maybe. Otherwise, seriously consider upgrading your lenses first.