Strictly speaking no; but it depends on the rood covering, it's more of a problem if the roof covering is slate, less so if concrete roof tiles. Slates and tiles are not designed to cope with concentrated flows of water (as you might expect from from a downpipe in a downpour), the result can be water ingress in heavy rain as water is drawn up the roof covering by capillary action. If the area of the existing roof being drained is small then it may be sufficient to provide a lead "soaker" at the bottom of the downpipe to disperse the water flow over. The solution adopted by your builder is fine for small roof areas, such as porches, canopies and the like.