I am an employer with lots of part time employees that have other jobs. One of them is on paid maternity leave from her full time job. She has asked to come back to work for me while she is on maternity leave. I am concerned that if I let her she will get in trouble with her other job. Is she allowed to come back to work for me and still get maternity leave from them? I know with a new baby she needs that job more and don't want her to risk it. If anyone knows the rules surrounding this please let me know.

That's up to the policies of her other employer. The law does not address it one way or the other except to say that employees who are on maternity leave and/or FMLA cannot be treated differently than employees who are on non-maternity medical leave/non-FMLA leave.

I wouldn't worry what her other job will or will not do. That is between her and her other employer. If she has been medically cleared to work for you, let her return. It isn't your responsibility to make sure her other employer is ok with it.

Most disability policies, too, allow employees to earn up to an amount before it would count against the benefit. Her "other" employer may not be amenable to offering flexibility or part time work so her policy does come into play and on her, no liability on you to ask or clear it.