Can my ex-wife's boyfriend (now ex-boyfriend) claim my daughter on his taxes. She doesn't file taxes. Shouldn't I be able to claim her since I provide 50% support

She doesn't work, doesn't file taxes. I just found out that she is receiving welfare/SDI for the past 6 years. No reported income as she works under the table. They also have a child together so he should only claim her and not my daughter.