Children over 18 and divorce

My ex-husband who is the non-custodial parent was allowed to claim the 2 older children (I claim the 3rd who is the youngest) on his taxes when we got divorced and they were minors.  Flash forward to now and the oldest turned 18 in Jan 22 - she has always lived with me full time and I am the legal guardian.  She is now a student and dad is not paying anything towards college.  I have not filled out any paperwork allowing him to continue to file with her as a dependent.  I went to file my taxes and he already claimed her so it was rejected. 

 

Does he have any right to her as a dependent?  It is impacting her education deduction as well.  

 

I have another child turning 18 this year and I fear the same scenario-- any help is appreciated.