Is it common for a person who drinks a lot to wet themselves and have no memory of it?

Yes. Wetting when drunk is very common usually amongst girls in late teens and twenties. most girls have at least one accident and some have multiple accidents - this can lead to deliberate wetting in some cases.

Additional:

You can not stop her from drinking. Only she can make up her mind to do that. You can however learn how to deal with it in ways that may benefit her. And, please keep in mind that we who have a loved one who drinks can become more ill than they if we allow it. Get involved with Alanon. Learn what will help her. Because what our instintcs tell us to do is actually going to worsen the problem.

It is not easy, but we must....