Sporadic loss of wifi connection on 13 inch MBP

My brother has been having sporadic loss of wifi connection on his macbook pro that he bought this August. His friend's computer (windows) seems to work well while my brother's wifi sporadically ( once-twice a week) stops working. Sometimes he is in the middle of writing an email and losing a connection, thus losing the mail itself. Any idea why this may be happening? What can he do to isolate the problem?

Thanks!