I don't serve peanut butter and am careful what I bring into my home as not to tempt fate with food allergies. When we have the young ones we never know if they could have their first allergic reaction with us. One of my first dcks was allergic to tree nuts and I did have an epi pen on hand. Never had an issue because I was careful and continued to be so after he left my care. Since I updated my child cpr last spring, I now have a childs epi pen in my first aid kit. I spoke to my insurance company when I had the child I just wrote of and they stated an epi pen in the house and my policy does not require an amendment. The cost is a write off, I have piece of mind should there be a "first" reaction in my home and the parents feel safer because I have it.
Last word, yeah the Mom should be hung out to dry for not protecting her child and not letting you know up front her child had this allergy, however, I would keep him (and his epi) as I wouldn't want to send the message to the child he is too scary to care for or not worth the effort to put in place preventative measures. JMO