He wasn't going to comment on Severus' reticence about Faustus' mother, that was something between Faustus and Severus that would have to be settled, and simply said, "I'm not sure if it matters if what they're saying is true. No matter what happened between your parents, they got you out of the situation, so everything was worth it. I'm sure they would tell you the same thing."
He paused, and then took a breath. Hopefully Severus would take offense too much at what he was going to say next. "I didn't know your mother, but I do know she was young, and probably foolish. We're all pretty foolish at the age she was when she was pregnant with you." He smiled slightly.
"And your father would never have done anything the students are saying. Not only because he doesn't need to, but because he's a good man." He didn't know exactly what Severus had needed to do as a spy, but despite Faustus' maturity, he was still a little boy and didn't need to know particulars just yet.
Terry was about to say something else when he heard a knock at the door. He saw Faustus' face fall and the hunch to his shoulders. He knelt and gave Faustus a quick hug. "Don't worry, okay? Your dad loves you." He let go, and stood to answer the door.
"Severus," he said warmly. "I think I have something of yours." He stepped aside to let the other wizard in, hoping the older man's temper would stay checked while he talked to his son.