Arnauld is an Addicted gambler.

He shouldn’t be your banker.

The ad hominem only works if 1 it is true and 2 it bears on the issue of the conclusion. If you judge it not to be either true or relevant, it’s irrelevant. Even if the premise is true, there needs to be a link between his gambling and a conflict of interest that could show a serious opportunity for Arnauld to only serve his interests not yours.