Should I feel guilty?

I have a 12 year old step son. I've been in his life for 3 years. When I met him he had an imaginary friend and still had him up until recently. He doesn't have any real friends and gets bullied a lot at school because he has a huge birth mark around his eye. His mom abandoned him and comes in and out of his life. She usually leaves again when she gets a new boyfriend.... She's back again. Well basically he killed off his imaginary friend... Or more so ... His mom did? When she was driving him back to us he said she ran him over. Well.... He didn't seem to upset about it. We had family therapy and one of my husband's rules for her coming back is she has to so family therapy. She came and she sent my step son to play and spoke to us about his imaginary friend. I said he didn't seem too upset about it and she said he killed him off on purpose. Then she says to me "He talks about you a lot". She showed us pictures he drew. It used to be of him and this friend. Then he started drawing him and me. She told me he was lonely and friendless but he found love and a friend in me so he isn't so lonely and no longer needs this friend. His bio mom got mad and stormed out.... Now I feel extremely guilty!