He/she needs to e-mail abuse-at-hotmail(dot).com with the following details:
-> Exact error message received.
-> Date/time this began occurring.
-> The name the account is registered under.
-> The alternate email address currently on file.
-> Account specifics, such as list of contacts, date of birth, recent sent mail, et cetera that helps prove account ownership.

In short, the more detail the better. A Microsoft support representative should respond within one business day, but in some cases it may take longer.

NOTE: It is best to create another Hotmail account to submit the request as requests from non-Hotmail accounts have been reported to generate erroneous autoresponses regarding spam.

Hope this helps,
John