Valiant Bolier hekp please?

My BIL has converted a 2 bed bungalow into a 4 bed house and done all work except lectrics. Gas and boiler not touched as not Corgi regged.
Last weekend he put hot and cold water pipes up into new build and now when he turns the cold tap on, it works, but, when turning off the tap, the boiler kicks in i.e pump runs for a few seconds but the boiler does not flame up. Now, the fact it was working b4 he put extra pipework in is worrying especialy as the cold is off the mains and nowt to do with boiler. or is it?

Hot water is fine with no probs.

Any help approciated. thank you in advance