If by that you mean can he sign everything as though he'd installed it, no, it would be illegal, because he didn't.
If you HAVE the situation of a boiler you need to be made “legitimate” then a Registered engineer technically would have to remove and reinstall it, and flush clean and refill the system with inhibitor. Then he could say he'd installed it, notify it to Corgi and Building Control, and fill out the Guarantee forms.
Also be aware that if you don't have all the paperwork in place, you can expect to find that manufacturers' warranties, and service contracts such as British Gas 3 Star, are void.
There is more here , issued by Corgi to registered engineers.
