HK 5.7 huh? You sure it isn't the FN 5.7 (Five Seven)?
If the latter then yes, they are very fun. If he was serious about it being an HK, then good luck finding one because I don't think they exist
The most fun handgun that I have shot is a Ruger Mark III target with a red dot sight. Lots of cheap and accurate fun that little pistol is.