wisteria pruning

I hard pruned a wisteria for a friend last year, now there is new growth which has shot up on a single stem like there's no tomorrow, (there is another shorter stem growing from the base) but, I don't know whether I should trim back the main stem and if so when, its well over 7ft, and grew 4 in a month, should I leave it or cut back, it does looks v healthy, any advice would b helpful, it's not a plant I am familiar or confident with. Thanks J