Your dad is somewhat atypical for ISTP. ISTPs are generally quite good at being patient and calm listeners to people that need to vent a bit.
Bottom line, until both partners feel that something needs to be fixed, nothing will get fixed. Period.
I can tell you that doesnt sound anything like me. Are you sure he is ISTP? I have no explosive tendencies and have no problems hearing about my wifes bad days. ISTPs are really like ESTPs when it comes to be laid back and easy going. He is probably socionics ISTJ and she is ENFP which is conflicting relationship.
I am an enfp and was married to an ISTP for about ten years. He also refused any sort of marriage counseling. "He" didnt have any issues.
There is something I do not understand about ISTPs honestly as they seem to come in two flavors-one more like Poki and many others on the board-and others more like your father and my ex husband.
My ISTP did not want to listen, or talk-not just about problems-but anything at all. "Are you still talking?, Why are you wasting oxygen?" He was so introverted that is was very hard for him to sit and listen to me. I was forced to play a strong J role and take care of all the financial, childcare, housecare roles. It is one thing to do this as a single mom, but another when you are forced to through anothers lack of interest.
The N/S was the hardest thing. We lived in two different universes and could not talk about things. I turned elsewhere to friends to keep my mind alive. At home I was forced to be an ISFJ as to not annoy him.
He also could be "tough" with ripping sarcastic, hysterical humor. He makes Edgar and the entps look utterly cuddly
I learned to roll with the punches but had to cover and protect myself to do that-which meant i could not feel happiness or love for him either. I think this may be an enfp thing.
I had to become something I was not, lie about what I was, and hide my true nature to fit his needs and keep the relationship happy. I dont know if this match is a good one.
What works is to issue a command-Move OUT. Suddenly this will pop the istp out of thier hole in the sand. Once there, dont relent. Make them leave, make them learn to feel what it is like to be alone. Then they want to return. I made the mistake once of letting mine return. He returned better.
If she does this, he will come back much more loving and will have an appreciation for her feelings, potentially even being very sweet, once he realizes what it is like to be alone..
However I will not do take mine back again, as I should not have to sacrifice my soul, by essence, the parts of me that are most sincere, my mind, my abstract thought, to match another person's needs. The same patterns would crop up, the same problems. I could never be utterly honest and open with him, as he disregarded those parts of me as useless and stupid.
Here I play, run rampant, and get to be a bit more crazy than I ever am IRL, but it is so liberating to be true to what you are.
I am too happy being alone, to sacrifice that freedom.
Sorry to be negative, but perhaps this is your mom's chance to develop into something more.