Successful mentors ask good questions in order to have a clearer understanding of the situation.  Not all need to be asked at once, but here are some examples:

• How did you first meet this other person?  Were you ever close to him/her in the past, before you were married? 
• Do you ever hide this relationship from your spouse?
• Do you confide things to this person about your marriage that would make your spouse feel uncomfortable?
• Do you and this other person exchange flirtatious humor?