give (one) enough rope
If you give people the opportunity to do something wrong or detrimental to themselves, they will usually do it; one does not need to interfere to bring about someone's downfall. The full version is, "Give (one) enough rope, and (one) will hang (one)self." Don't interrupt him with questions. Just let him keep talking and he'll incriminate himself. Give him enough rope and see what happens.
give someone
see other entries beginning with give someone, or under give one, or look up the expression by its other words, as give the benefit of the doubt.