If he (she?) can afford it, it might be worth taking a trip out to where these things are grown in italy. (sorry, don't know where) the reason for this is apparently the climate which makes for faster growth, but is nt tropical enough to affect the hardiness of the specimens. You may have noticed that topiaries of all kinds are cheaper now than they were say five years ago, and if your friend hasn't already done so, it might be helpful for them to do some research on how much the product is going to cost to produce and whether it will be saleable at this price. Lovely idea though, I quite fancy it myself (no, I am not going to, it sounds like very hard work!)
All the best