Roommate rent dispute?

3 months ago myself, my boyfriend, and 3 other friends moved into a 3 bedroom apartment. (5 people) My boyfriend and I share a room, another couple share a room, and the remaining has his own room. The girlfriend of the other couple brought a puppy home and we told her we weren t allowed to have pets (by our lease agreement) she lied and said she was just fostering it. 2 weeks later she said she was keeping it, asked our landlord and he said no. She then decided she was moving out in order to keep the puppy. Only 3 people signed the lease, she was not one of them. The other 2 roommates said she doesn t have to pay rent (even though legally she d have to if she signed the lease) and they told my boyfriend and I that we have to split the cost of her portion of rent. Her boyfriend and BROTHER told her she doesn t have to pay. I believe she should still pay her portion since we all agreed to this 16 month lease, or her boyfriend and brother should be responsible for cover her portion. Am I being unfair to think this?