Ludwig,
Has your friend spoken to any insurance companies about his purchase?
I can't say for sure, but I would expect that a requirement for insurance (assuming he's planning on getting some, of course) would be for a reasonable amount (50 hours, maybe?) of tailwheel time. If that's the case, and if he's not doing so already, it may be worth him completing his PPL on a tailwheel aircraft - there are plenty of places he can do this if he looks around. Even if it's not an insurance requirement, or he's planning on flying without insurance, I would guess that getting as much tailwheel experience as possible would be a benefit.
FFF
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PS - My old PPL instructor owned a share in a D9, and loved it. He used to race trucks along the M4 in it - he reckoned he could sometimes beat the trucks, too, if he had a good tailwind! Although I haven't had a go myself, I have no doubt he will love his aircraft as long as he is suitably qualified and experienced to fly it.