Personally, I don't concur with a number of responses on this thread.
Its a very decent bike. Forget stuff like it not having boost spacing or anything on the forks, that's a non issue for 95% of normal people and I'd very much doubt you'll have issues getting spare parts if anything needs replacing, but the reality is, its doubtful you WILL need to replace things like forks etc anyway.
The only thing you do need to be wary of is the battery, however all the evidence shows that Yamaha haven't skimped with the quality of the cells, so if the owner has stored it properly, chances are you're going to be fine for a good few more years, despite the age. Look for posts here by George Henry about his yamaha haibike and the age of his battery. And I'm also talking from experience, as my wife has the female version of the bike in question here, and that's even older and has done something like 5000+ miles and the battery is still good. I'm sure it must have dropped in capacity somewhat, but the fact that its not easily quantifiable speaks volumes.
Ultimately, the best advice on this thread is something you indeed seem to be doing, which is borrow the bike and try it. So fully charge it up, and ride it until the battery is empty, just keeping an eye on whether the charge does something strange like drop from 40% to zero in one go. If it doesn't do that, and its a nice gradual(ish) decrease with a decent range, I'd say you're good to go, and given the price and availability of new ebikes, well ... I think your mate is going to get the £1000 he's asking.
if the battery is knackered, then there are 3rd party ones out there for £300-400, so negotiate with your friend who's selling.
But if it was me, I'd be VERY interested in getting this bike.
Thank you for a very honest reply.
I did buy it in the end and am absolutely loving it, battery life is fine and it flies along. I use it every day.
The brakes were a bit 'soft' so I changed the disks and pads and all good now. I have noticed I need to be careful of braking distances being longer than I'm used to due to the extra weight and momentum.
Mid drive is 100% the way to go, it feels and rides like a normal bike but as if you had a pro cyclist and not me on it
Thank you to everyone for taking the time to reply, it is very much appreciated.