It would depend on what you want to get out of him answering the question. It may be more harmful to get these details as later on you may torment yourself with this information. If you're wondering if he is honest, then wanting to get a general idea about this other person is reasonable.
It may be healthier to focus on repairing your relationship because talking in detail about this woman may elicit thoughts in him to think of her rather than work on himself.
Even if this woman is the total opposite to you, it does not imply that you had done something to lead to his unfaithfulness; it was his choice and he's used the opportunity. It does not either imply that the look or qualities she possess is something he'd rather have.