Mother with AFib. She complains of frequent urination at night and leg muscle cramps with tight sensation at ankles upon waking. Any ideas?

Take her to her doctor. She needs checking out. I expect that the doctor will want to run a few tests to see how her kidneys are functioning and what's going on with her electrolytes. It may be that something has gone wrong and needs attention, or that her medication could be changed or adjusted.

If the doctor is not one she knows well, make sure you take all her records and her regular prescription with you - and don't let her go the appointment on her own! I'm sorry to stereotype "mothers with AFib" but I know of too many who assure the doctor that they're doing fine and completely forget to mention the key problems.

If this has been a sudden change, I wouldn't wait - get her seen as soon as possible.

She should keep her feet elevated whenever she sits during the day. If she allows a lot of fluid to collect in her feet and legs during the day her body has to work all night to get rid of it. Ask her doctor for blood work to make sure her salts are good and she’s not showing signs of dehydration.
Is she getting up by herself at night or is someone tucking her in? Her bedding may be too tight over her feet.
Is she on Lasix? If so, pay attention to when she takes it. How about her diet? Is she watching her salt intake?
Does she track her weight daily to monitor how much fluid she is gaining? She should also get some exercise every day to help the valves in her legs.
I hope you are able to help her find relief. A bit more information would help the nurses on the forum drill down on the problem.

urination is more frequent with increased salt intake during the day; is there swelling in the ankle or only tightness? If there is swelling then there is good reason for the tightness. However, the cramping alone will tax muscles to the extent that they are sore the next day until they are used more. Cramping in the night often due to dehydration. Make sure there is no alcohol intake, as that can make it worse. It is worth getting a blood panel now, because often in any senior, especially those who have to take diuretics for CHF, there is a resulting low potassium which will cause some vicious cramping in the night. Some people are more naturally prone to cramping. My Dad had it most of his life and I get it as well. One of my worst fears is being unable to get up and work out cramps. They are awfully painful. Hope you solve the mystery.