Obviously. virus comes to his computer!
If it is just the SPAM which does this it could be whatever is in the message. Have him just delete them instead of trying to open them. You wer not specific in whether this happens with ALL SPAM messages or just some. Also is this mail which is legitimately tagged as SPAM or messages sent to his SPAM folder which is not SPAM which he wants to move out? Also to give you more assistance we need to know what application this is happening with, is this outlook or some other mail program or webmail? Please provide more details including version and service pack level. Thanks!