What do you do when involuntary movements prevent her from sleeping at night?

My husband "jerks" when he sleeps on his back. Why, because he has a deviated septum that keeps him from breathing correctly. Meaning he is not getting enough oxygen to his muscles. Has no problem sleeping on his side. I agree, sleep specialist is needed.

If this is restless leg syndrome, you need to get her to a sleep specialist. There are meds available

Much of this depends upon the reason for the movements. Recently there has been some success with very low dose antidepressant medications. Do speak to the doctor.

You may find this helpful

https://www.mayoclinic.org/diseases-conditions/myoclonus/symptoms-causes/syc-20350459

There are medication that can help with involuntary movements.
Talk to her doctor it is possible that a medication adjustment is necessary. Or a medication added or removed.