My fiance wants to hide our engagement from his mother to keep using her money until graduation

We have been together for almost five years and we got engaged this past NYE. His mother is mentally ill and has major PTSD from her time during the war. She hasn't liked me since high school when I broke up with my s/o for three months during my parents divorce. She said I needed to apologize to her for "having to repair what you broke". Alright so I tried to make amends and wrote her a letter about how much I love her son and apologized for any inadvertent "hurt" I caused her...No positive responses from that and honestly I do not want any commentary about how to strengthen my relationship with my future MIL. Both my fiance and I agreed she will not be in our lives or our children's lives following his graduation. He is receiving money from a prepaid VA529 and her GI bill. She, before we even talked about marriage, threatened to pull this funding if she found out we were engaged. While it seems like a simple answer she abused him as a child and continues to emotionally abuse him to this day. She is controlling and will not allow me to step foot in his childhood home. She refuses to be in the same room with me and threatens to pull her money away whenever he doesn't follow her demands to a t. He knew it was the natural step to propose but he wants to keep hiding it until December when tuition is all paid. I told him I would help with his debt if he wanted to be independent prior to graduation but he thinks it is necessary because he will use what money he saves from no debt to put a downpayment on a home for us. I get why this makes financial sense but I'm having a hard time with this. We have told literally everyone besides her, my future father-in-law even knows! How should I help him make the decision to cut this woman off?