Dog Poop

My girlfriend and I adopted a 6 year old beagle together. The dog, Lucy, usually stays at my house but today when I went over to her house my girlfriend let me bring her. The dog doesn't like being left alone though she does fine when she is. When her dad got home he asked us to go to the store with him which meant that we had to lock Lucy in my girlfriend's room because she likes to get into the litter box. After 2 and half hours, we returned to find that Lucy pooped on the carpet. My girlfriend told me that Lucy isn't trained well enough enough and that she needs to learn better. I don't think it is the dogs fault. Who's right?

Anastasia McKnight