I just found out my daughter's marriage is in real trouble. How can I encourage her and her husband to work through their problems?

You should encourage them to seek counseling from a therapist or clergy. Do not try to help them yourself - this will not end well.

Never look like you are taking sides
IF you do volunteer advice always be like ''on the one hand you have to see [this] from his perspective but he also needs to realize that on your side you are right about...
Balance everything out and make it from an outside 3rd party view where you don't become personally involved.