Email was sent by user but he claimed his email account was hacked

Hi There,

i have a situation here and i would like someone from the Microsoft to email me directly as it is a sensitive issue which could be leading to a civil law case.

Is there anyway we can prove that the email was sent by user and not being hacked, as according to Wikipedia; hotmail are using a single-use code; hence it means the code is being used by one user only and even if he or she is signed on a public computer, a single use code helps to keep account information secured.

I really need some assistance.

Thanks

J