Sidearm throwing is a hard habit to break.
We tried thromax with one girl, but she hated it (it's not what you call comfortable, but it does what it advertises...). Personally, I like to tape balls with electrical tape to make a perfect circle around the four seams. Then have the girl put her fingers on a seam with her middle finger on the tape, and make sure her thumb is on the tape. When she throws, a solid black line should be visible at "12-6 o'clock" or close.... If not, she is not throwing properly and it is easy to see. It's impossible to throw sidearm and get a 12-6 rotation. Visible feedback is a good way to teach.
They have to really start with fundamental throwing form, accentuate the motions (big follow through) and do it slowly, then build from there until she can throw quickly with comfort. It shouldn't take all that long to make the overhand throw feel right if she is willing to work at it.