I ran into this same problem but the pipe was really hard to reach. Replacing the elbow would have meant moving a joist out of the way. So what I did was get a scrap piece of pvc pipe just slightly larger, cut a square patch from it, warmed it up with a heat gun, using a wad of rags to keep from burning my hand, I wrapped the patch against the leaky area till it cooled off, thus matching the profile. After that I glued my newly formed patch with pvc cement.
That was over 10 years ago and I haven't heard back from them, presumably because it worked.
Note that Glenn's advice is best, but I just thought I'd share this story.
Merry Christmas!