I have always said that one of the worst things about sports is that, in almost all of them, the specific movements cannot be replicated in the gym. You can work out as hard as you want and as carefully as you want, but there is no real way you can replicate the impact and mechanics of a FH in the gym.
You have to go on the field and hit the actual stroke.
After such a lengthy layoff, she is still probably far away from peak tennis form.
Keeping the distances between the pros and us, I am going through that right now. My tennis elbow is getting better, but it is only getting better by going on court and hitting balls. All the off-the-court therapy means nothing if you don't go out there and play. No exercise is going to simulate the swinging motion AND the impact of the ball on your racquet (which goes straight to your arm).