The ulnar nerve is a branch of the bundle of nerves that start in the neck and come down the arms. It's located on the inside of the elbow. It supplies messages to muscles in the forearm and hand. The ulnar nerve also provides sensation over the fourth and fifth fingers of the hand, palm, and the back of the forearm.
The ulnar nerve can get trapped inside the soft tissue around it. The most common site of entrapment is in the elbow. The result is a condition called tardy ulnar palsy. This problem was first described in 1878. Tardy refers to the fact that the problem is delayed. It doesn't happen until thickening or scarring of the tunnel around the ulnar nerve closes down on the nerve.
This pressure on the nerve sets up the symptoms your mother is having. At the same time, normal nerve signals may not be getting through. It's probably time for a follow-up evaluation to see what's wrong. Waiting too long can result in permanent damage. Even as it is, nerves can be slow to heal an without full recovery.
Management with conservative care may be all that's needed. If not, then surgery is a final option. Your mother's physician will be able to advise her best.