If he has a 4x4 key he can use it without steam. If he has 6x6 or 5x5 or any other variation on the other hand, he needs to send a mail to sup[email protected] asking to have his current CD key converted.
He should definitely send a mail to Steam support as well, because this is an issue with Steam, not Mount & Blade.
Go through https://support.steampowered.com/kb_article.php?ref=2204-OPGC-4862 properly.
Maybe this as well https://support.steampowered.com/kb_article.php?ref=9828-SFLZ-9289