You're not the a*hole, I think it's natural to get annoyed because it's wasting money on food that's still good by dumping it in the trash. To me, if you have a condition that could get better with therapy and it's not like the end all/be all of mental conditions, then that makes the other person responsible for themselves because you can easily use mental illness as an excuse for doing anything to anyone. Nobody can moderate you except you, so to me, people need to find a way to get help so they recognize HOW to deal with OCD related issues rather than just not dealing with it at all.
It's kind of like me with my panic disorder, I had panic attacks in the past and when it got to that really dark point, I was desperate enough to do anything. 8 years later and I've managed well through acceptance therapy and exposure therapy/changing my mind frame around quite a bit.