I have to agree with the others. Probably need to nip this in the bud right now. My mom has this friend and she brings her little one over to my mom's house and almost every time she comes over the dog pees and/or poops on my mom's carpet. My mother is really a nice person and wont say anything about it. Last time it happened the woman states, "She doesn't do that at home". Ok, that may be true but since you know she is doing her business in my mom's house, pleeeeez stop bringing her over or else keep her in your lap. I really hate it when people take advantage of nice people and their hospitality.
Bottom line, it's up to you. If you like cleaning up after Brian then go ahead and keep doing it. If you think you want to be friends with Brian's mommy then talk to her about this little issue. But, if you truly don't care for Brian's mommy then you should end it now. JMO
