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The amount your fiancee pays should be determined by upon a comparative analysis of his income and the income of his Ex. So, with her inheritance, he should go back to court with a request to "modify" the court's orders concerning the amount he pays.
Regarding, continued payments because the child changed her degree program, first re-read the agreements and/or court orders to see what you can discern from them concerning this change in circumstances, and if in doubt, you can petition the court for a clarification of his responsibilities since the alleged change in degree program seems to require him to pay longer that he otherwise would have.
In sum, this needs to be back in front of a judge for clarification and modification. Go ahead and take the intiative.
All the best,
You need to go to the court house and request the forms available for petitioning the court for a modification - your requests for change need to be written and file din court before you appear there in response to whatever action his Ex plans to take -