Provided you do fill in the amount line and that the signature on the cheque is one of the signatories on the account, there is a good chance it will clear (forgot to mention the money needs to be in their account!).
Many company cheques provide space for two signatures, but only actually require one - in some cases there is a limit on the amount that a single person can sign for and spending above that needs two, but quite often that's for amounts much larger than 5k.
Personally, I'd lodge it and see what happens.