Yeah, my mom calls me Punk and Cookie and a million other things that have no bearing on my legal or professional life. Also my grandmother has gone by her middle name socially all her life and has zero problems switching into a legal context--if she needs someone to verify her identity, she picks a friend or family member who knows that Firstname Usedname Lastname is her, problem solved. Unless you go into this guy's letter with the preexisting belief that everything is his mom's fault, it really doesn't come out with that answer.