If you generated a key pair you should have two files id_rsa and id_rsa.pub (note that id_rsa may be replaced by an other name if you specified it).
The public key (so the *.pub file) is the one which is meant to be shared so this is the one you should send to your customer.
However note that even though this file is public it is important that your customer receives the one you generated. If someone were to temper the file they could impersonate your server with the security issues this implies. So it is up to you to decide of a safe medium to share this file: This could be a physical USB key given from hand to hand, a secured messaging service or any other secured communication channel you have with your customer.