Being an Hk nut also, I can tell you a bunch about the SMG if 'guy' divulges any more specific details.
Single fire MP5's are not 'standard' persay; most in circulation have Select fire groups of varying burst capability.
A single fire FGC fitted to an MP5 would have been an after thought.
Being that you're in Kalifornia, I'm hard pressed to believe the legal possession of a true MP5.
More than likely, he has a long barrel HK 94, which is the Carbine 16" version of the MP5; There are much greater numbers of these in circulation, and tend to be easier to 'arrange' in non-gun-friendly states.
The first question you should ask him is how long it took him to Class 3 the weapon; you'll know right away if he's talking out of his butt.
If it's not properly Class 3'd (and we all know how long it REALLY takes to get that sort of thing) then more than likely it's either a 94, a SW clone, or it's not a real firearm. (you'd be surprised how many chumps try to pass off Airsoft replicas as real steel.)
Obviously, proof is in the pudding; Ask for him to show it to you.
Then subvertly search for the Class 3 engraving. If he can't show it to you, run like hell, and never talk to him again.
(and never admit you saw the weapon)
E.
(edit - wow, I need to type faster, half of you answered the bits that I typed up! LOL )