A friend recently said to me, “I owe you due diligence”. I told him that “due diligence” does not make sense i

you can owe someone the duty of due dilligence but you can’t actually owe someone dilligence. Semantics, sorry.

Not without more context. If you two were in disagreement about a fact (not an opinion), he knows that he’s right and you know that you are right–he may be saying that he respects you enough that he needs to double check the fact, even though he knows he’s right.

Yeah, he’s probably misusing it. I can’t imagine any other way ‘due diligence’ has anything to do with friends. It’s usually used in business law, and basically means that you did your homework before you made the deal.