WF,
In defence of the JP. It is a little slow and not really a 'fast jet' but is a very good basic jet trainer that with proper training can be flown SAFELY by an average PPL.
The Mk5 can do the 300 kias low level so is a good jump up from a light single. As other comments say - go one step at a time. A good grounding on the JP will stand you in good order for other jets.
I know the guys at Swords Aviation (see
www.swordsaviation.co.uk) and I'm sure they can get your friend up for a trial flight.
PS. In my opinion the best JP is the Mk4!