I keep telling her she will GREATLY benefit from this degree down the line but she can't see that right now. How can I encourage her to buck up and get through the next 18 months and then things will prove worthwhile.
Here's your problem. She is really upset and feels like you butt into their lives and sent it in a direction that she's not happy with and you are telling her that you are right, she is wrong. You're saying you'll see that I was right.
Like others have said, back off. Listen, but don't say anything except to offer to help. If it's not babysitting, can you do something else?
I went to night school and worked full time. I could barely manage life and a relationship - can't imagine having kids. Most people I know who did have kids had terribly difficult marriages and everyone I know who had a baby in law school dropped out - male and female. And, to top it off, many people get out of law school and hate their options. Hopefully, your son and his wife will fare better.