My 2 yr old son just drank after gma with cold sore :(?

My 2 yr old son must drank after my mom and she has a cold sore I walked in the room and he was already drinking out of her cup :( I'm not sure why my mom let him she had always been goid about it my brother and I have never got one.. Now I'm freaking out hers is to the crusted over anythingI can do opinions