What is the name of your state (only U.S. law)? Texas
My daughter found out that her "husband" was still married to another woman when he married her. The other wife has filed for a divorce. Is it too late for my daughter to seek an annulment?
Maybe.
Rules on TX divorce and annulment are here:
Texas Annulment
There are two issues you have to look at carefully:
1. If your daughter's husband gets divorced before your daughter files for an annulment, then the marriage automatically becomes valid (see section on void marriages). If she's going to do something, she'll have to do it quickly.
2. If your daughter doesn't do something about it quickly, it could be that she has condoned the situation legally. I don't believe that has any DIRECT bearing on getting an annulment, but it could prohibit her filing for annulment on grounds of fraud (if she doesn't move fast enough to file on grounds of his other marriage).
She should see an attorney.