I agree that her posting about feeling low on Facebook and getting a call from this single male parent is bothersome. And equally bothersome that she made a connection through a play date with the children taking your child to this single male parents home.
So find out the day and time and go with her and make it a point to meet this man.
I did not let my child play with other children unless I first meet the parent. You don’t know who this guy is. Nothing wrong with finding out who he is and meeting him.