Mary Jo Eustace, who split with the actor and reality TV star in 2005 after he fell for Spelling on the set of the Lifetime TV movie Mind Over Murder, tells PEOPLE exclusively that while she wishes McDermott, 47, the best, "My main concern at this point is the well-being and health of the son we share."
The exes, who divorced in 2006 after 13 years of marriage, are parents to son Jack, 15.
Eustace, 51, a Canadian actress and TV host who wrote about the break-up of her marriage in the 2009 book Divorce Sucks, tells PEOPLE, "I wish Dean only the best in his struggle to overcome the demons he has been wrestling with for many years, so he can become the father and the man I know he wants to be."
"I am also hopeful that from this point on there will be a dedicated effort to deal with this matter privately and respectfully, for the sake of all the children involved."
McDermott – who also has four kids with his wife, Spelling, 40 – has been the subject of cheating rumors since Christmas. He confirmed to PEOPLE on Thursday that he had checked into rehab.
"I am truly sorry for the mistakes I have made and for the pain I've caused my family," he told PEOPLE in an exclusive statement. "I take full responsibility for my actions and have voluntarily checked myself into a treatment center to address some health and personal issues."
Reporting by JULIE JORDAN