The best way to get her weight up if she's very thin is to offer smaller meals more often.

Instead of stuffing the snake with a large prey item every 7-14 days as you normally might, keep the offering small, but go every 5 days. Her digestion will have an easier time with small items, and thus you won't overfeed her on the short schedule.

Then as the weight returns, gradually go bigger prey and less frequently.
Best wishes with her.

Gale