Your wife is volunteering for this organization? It seems like that's a good explanation, "My wife and I are getting divorced, so I need to step away from projects she is involved in."
Given that you have no evidence this employee is involved with your wife now, that means any future involvement would be post the filing. I'm not sure it's a fireable offense unless this is a religious institution.
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16