what would you do if you caught your wife in bed with your dad?

I find myself imagining what would happened if such happened to me. A friend and I were discussing yesterday and he opened up to me about this issue that has been on his mind for two weeks.

My friend had a little problem with his finances so he had to move back into his parents' house for a while till he could get back on his feet. He has no idea how long this has been going on since his wife does not have a job. He got home early than expected and actually met his father making love with his wife. The issue has been kept low but he said he feels like killing both of them anytime he gets the chance.

What would you do if it were you?