Full Idea
Judicious use of an opponent's assumptions is quite capable of producing a perfectly reasonable ad hominem refutation of the opponent's thesis.
Gist of Idea
An ad hominem refutation is reasonable, if it uses the opponent's assumptions
Source
Verity Harte (Plato on Parts and Wholes [2002], 1.6)
Book Reference
Harte,Verity: 'Plato on Parts and Wholes' [OUP 2002], p.35