Just say no. Return the next thing she sends to you with a little note saying that you simply don't have time to help her out again but you would be happy to show her how to do the repair herself. You are definitely on the right track!
I teach sewing in a high school, the first year we had the class staff members frequently came by with their repairs asking us to fix stuff. This is a class mind you, my curriculum did not include miscellaneous side projects. And the poor kids, once their family knew they were learning to sew, they started piling their repairs on them. I personally don't do repairs and I don't do alterations either so I developed a lesson on "how to just say no diplomatically" so the kids could learn early how to handle these requests.
I tell people up front that I'm really lousy at repairs and alterations (and I am!!) and recommend a couple of dry cleaners that I personally use that do a really good job on that stuff.