I don't believe that the "witness" even saw anything illegal. I think that he merely saw them together on the couch and caused a stink. It doesn't even hint as to what the person may or may not have seen. A kiss? Holding hands? It sounds to me that the mother is trying to have the guy arrested just to get him out of the picture (random thought.. mother's boyfriend is the witness right? Could we have a little jealousy be at the core of this?).