Welcome to YT.
Im sorry that you were in this situation, what sort of person gives away a pregnant girl
Glad that you were able to get her to a vet and that they are doing well.
Seen as they had a yorkie-poo chances are he is the father but you wont be able to tell much until they are older.
Have you contacted the people who you bought her from and asked if she tied with their male? good luck with everything!