Can you dispute the charge if you let a friend use your credit card and do not sign the receipt?

No. The person who signed the card agreement (card holder) is completely responsible for any charges made on the card. The only exception is, obviously, if the card is stolen. Your only option is to get your friend to pay the amount and interest charges.

I forgot to finish my question. How could the card be used if you did not have it validated? If the card was sent through the mail without you returning any paperwork, someone would have had to activate the card. If your card was used by your friend with your knowledge, the debt is yours...period. The CCC might interpret your denial as fraudulent. I would be very careful in any future correspondence/discussion in the matter.