John,
I won't debate the point with you although I ran the merchant account for a company doing millions every month and might have a bit more experience than anyone here in dealing with Visa and Mastercard.
But I will reiterate, to do what you are doing places you friend's merchant account in jeopardy because:
1. He is not permitted to allow you to run charges through on his merchant account
2. No merchant may charge a surcharge for purchases if the customer selects a credit card as the payment method
What you are doing is not illegal however I am sure your friend will enjoy his relationship with you even more when an customer of yours who turns angry goes to his credit card issuer and tells them that either he/she was charged more for using a credit card or that he/she has never done business with the merchant and declares the charge to be fraud in which case your friend is either losing his merchant account, charged a few thousand dollars in a large non-refundable deposit, made to pay higher fees or has a higher reserve set for disputes meaning the next few sales of his are not remitted to his bank. One dispute and the red flag goes up and he is now the subject of an intense investigation.
Your friend will have no recourse against your customer, he gets to sue you and collect from you as you placed the charge. You see, he never had any business dealings with your customer. He would need to prove that he sold your customer something or admit that he is participating in a credit card fraud scheme whether intentionally of due to ignorance.
You really do need your own merchant account to become a reputable and respected merchant. Sorry, there is no candy coating the improper use of merchant accounts. It is not a question of rather or not you and your friend will be found out. It is simply a matter of when.