Have used YSC for mainly MOTs and spares and had no problems, the mechanics were helpfull and flexible ( on the loud pipe front if you nkow what I mean

)
i cant honestly say that I know anyone who has bought a bike from them, but you see quite alot of bikes with their logos on driving round the York area.
i've heard they are good for letting you take test rides and their prices shown on the 2nd hand bikes look quite reasonable. On the whole a big

for my dealings with them so on that score I would recommend them.
You know what is going to happen now, your mate will have a terrible time with them, I hope this isnt a kiss of death for them our your mate.
as a foot note, a few of the YSC staff, marshall a very un-official, mini moto session, on a local car park, where you ride what you bring, but the YSC lads mark out a short simple course, marshall it and gave sound guidance and advice to my two kids on their 1st time.
But I've not bought a bike from them and only for the reason that they didnt have what i wanted at the time.
Hope this helps
Rich