He dilapidated their already meager treasury "helping" friends who were in "need". The problem wasn't Conny's father lack of spine, it was his blind adherence to the "be honest on all your dealing" motto the Grail family has. His reasoning being "I made a promise and the honest thing to do is fulfil it even if I was being deceived." Something Conny herself calls out when she defines what "honesty" means to her.
The debt thing was "solved" with the engagement with Randolph. The "fake marriage deal" perk for Conny was paying the Grail House's debts.