That sucks. On one hand it's very disrespectful to touch/take/use another persons property without asking first, not to mention using it in a way that will damage it, but on the other hand, many people look upon a knife laying around the same way they would look at a pencil lying around, and they think nothing of picking it up and using it. To such people, a knife is just a piece of metal that probably only cost a few bucks.
Although I generally don't believe in carrying around a cheap loaner to hand out to other people, I'd recommend that you do just that when you're around your GF's dad. You never know when he might ask you to borrow your knife. And since he's your GF's dad, you might not want to flat-out refuse him, or risk offending him by asking "What do you want it for?", or by lecturing him on how to properly use a knife. Dealing with a GF's dad can be a tricky and sensitive matter, and it never hurts to keep your GF's dad as a friend.
My girlfriend's dad carries his own knives, so I don't have to worry about such things. But If I were in your situation, with a "dad" like that, I'd carry a cheap loaner around him.