My son's relationship with his ex-wife has become extremely strained.

My son is 29 years old.
Since my wife and I are currently divorced, I will refer to her as my ex-wife.
My son is currently serving as a non-commissioned officer in the military..
He has been independent since he was 20, and he never caused us any trouble growing up. Even as an adult, he has never asked us for money; he manages his own affairs, finds time to study, and even graduated from a cyber university. He is a very reliable and affectionate son, like a daughter.
However,
It seems my son recently needed some urgent cash and asked his ex-wife for 1.5 million won.
Even though she had the money, she told him to pay her back in installments of 500,000 won per month and got angry at him, making a huge fuss over a mere 1.5 million won.
My son said he felt very hurt by her and expressed all the grievances he had accumulated over the years:
1. My son started working part-time at 19 and managed to save 32 million won before enlisting as a non-commissioned officer at 21. Unlike kids these days, he was very frugal, but his ex-wife spent all that money without even telling him.
2. There was a time when my ex-wife spent about 2.4 million won—the small amount of pocket money my son had carefully saved from age 7 through high school.
3. I heard she borrowed 40 million won from him to start a new life with a man she met after our divorce, and she never paid him back.
= In conclusion, my son felt that when his ex-wife was struggling, he squeezed his own tight finances to help her out and encourage her, but when he asked for help during his own hard time, she acted like she was doing him a huge favor and made it obvious she didn't want to help, making it seem like she didn't love him.
Upon hearing this, she reportedly stammered and could only cry, as if she had nothing to say. My son was so angry after saying this that he told her he was cutting ties with her and hung up.
My son called me crying, lamenting that he is going to cut ties with his mother and that no matter how much he thinks about it, his anger won't subside..
Knowing my son was hurt because of his ex-wife makes me feel so sorry for him, and my heart aches seeing him cry sorrowfully all day.
If only she had married a better person, my son wouldn't have had to endure such wounds..
I sent my son 10 million won, telling him to forgive his mother and stay strong,
but he sent it back, saying he would handle things on his own..
Sigh.. As a parent, I feel so sorry that there is nothing more I can do for him.
Even though I am angry at my ex-wife, I hate her so much I feel like I want to kill her for making my son's life like this.
My son seems so deeply wounded that he has blocked his ex-wife's number and KakaoTalk entirely. Perhaps sensing the gravity of the situation, she tried to contact him, but when that failed, she called me, pleading for me to put her in touch with him..
My son seems to have made up his mind; he says he won't even treat her as a parent anymore..
Seeing this crazy woman pleading for me to let her contact my son, I want to scream curses at her, but seeing her cry and beg makes my heart soften again..
What on earth should I do?