A starting point would not be to refer to this as a "scam". You may be assured that Enterprise will never relent if he makes unfounded accusations.

If your friend did not obtain an inspection form, take photos himself, or have a written confirmation that the dent is pre-existing as he has told you it is, this is simply a he-said she said.

A starting point is to figure how much is involved. Don't provide CC information or anything else until he has a damage estimate. With that, he may hand this off to his insurance and be done with it. On the other hand, perhaps it is so small, that he wants to directly haggle with Enterprise to see whether Enterprise will drop the matter or arrive at a mutually satisfactory agreement.

No way to tell anything until he knows what is involved.