I cant tell you whether this person you are talking about is lying, only he can tell his story and knows whether he is telling the truth about how this disorder has affected him.

I can say that yes it is possible for people to have a happy childhood and have mental disorders such as DID or in your terms MPD.

if you read books that document this disorder you will find many documentations by psychiatric providers that have treated cases of DID where the client has had a very happy childhood, entered therapy because something in their adult life caused them problems ie a divorce, depression, stress..

Through clearing up those problems they find out they have had a happy childhood because they have DID.

DID is a mental disorder where a person splits off into 2 or more alternate personalities. in some people with this disorder all the bad childhood events happened while the person was dissociated into an alternate personality.

example

(using a mild and hopefully non triggering event to illustrate)

a child is in bed sleeping. they wake up coughing and smelling smoke, their house is on fire they fear for their life, they split off into being the child and an alternate personality. the child no longer registers they are in a burning home and could die. the person in danger is the alternate personality not them.

hours later the child wakes up safe and sound at Aunt Ethels house not remembering the fire, their fear nor their splitting into an alternate personality.

this child goes on to live a happy care free life because any time anything bad happens it happens to the alter not the child.

another example of how this can happen have you ever noticed upon yourself a bruise but have no idea how it happened, just one day you look down and see the bruise. you ask someone what happened and they tell you but you have no memory of it so to you its a situation of "ok if you say so but I dont believe it happened that way"

dissociation in children under extreme circumstances is what enables the extremely abused and traumatized children to continue living a care free life where adults under the same abused and traumatized circumstances would die or at the very least have irreparable problems.

most people around here with this disorder never knew they had a traumatic childhood because due to their dissociation abilities they did not experience a traumatic childhood. most around here enter for problems related to their adult lives and then through therapy discover they have DID and then through working on their DID they discover the traumatized alternate personalities and the traumas that they hold.

some survivors have been accused of being liars and much worse because they say they have happy childhoods but information about this disorder states it happens under extreme abuse and extreme trauma.

what those judging others as liars arent taking into consideration is the diagnostic criteria stating people with this disorder have memory problems and because of having a dissociative disorder they have no memory of the extreme abuse and extreme trauma they underwent as children. people with this disorder cant say they have a traumatic childhood if their memories of the trauma is held within alternate personalities.

btw - not saying you are making judgments in that last paragraph, in my own life I have had people do this to me, and I know other survivors that underwent the same judgement from others. I also have clients who have friends and families that make such judgments that the person must not have DID because they say they have had a good childhood.

my suggestion dont worry about whether this person is lying or not, or in denial or not. instead treat him with the respect and dignity that you would want to be given if you were diagnosed with a mental disorder that seems to conflict with your beliefs and life experience.