No.

I don't think its your place to believe or not believe your father and/or the source. If there is any uncertainty in the situation, that choice belongs to your mother.

It is ideal that the truth come from your father. But if your father's story conflicts with the source, then its not your place to decide that your father was the truthful one. I hate to say this, but this isn't your relationship. This isn't about you. This is about your mother and father's marriage, and she should have the final say in whom or what she trusts. She deserves the full picture, both sides, then she deserves to make that choice for herself.

To clarify- I think you're doing the right thing by asking your father to see if he'll confess and talk to your mother himself. But if his story doesn't line up with the source's story, you should go to your mother, even if you personally feel that he is telling the truth.