What kindof company is it? Does he know if it is limited by shares or guarantee or limited liability?
Any one, including a director, who signs "any bill of exchange, promissory note, endorsement, cheque or order for money or goods in which the company's name" is not "mentioned in legible characters" will be personally liable for the amount involved if the company does not pay (Companies Act 1985, s.349(4)).
He should check the invoice etc...
As regards liabilities, this should assist...
http://www.elbornes.com/index.php?section=arti cles¶m=33