An RFD cannot pass possession of an air rifle over 12FtLb to anyone who does not have an FAC for the said firearm.
If he does then he is supplying an unlicensed firearm, an offence, also the recipient of said firearm is now in possession of an unlicensed firearm, another offence. If such a person tries to leave with an unlicensed firearm (their own or anyone else's) that's a whole different league of criminality you just found yourself in. Don't forget, your face will be on CCTV, any threats or attempts to take a gun by force will be recorded.
I am an RFD, if somebody brings me a faulty gun which chrono's over 12FtLb then it is immediately locked in the cabinet for disposal, repair or until the owner has applied for and received the relevant certificate to possess the gun as an FAC (Section 1) airgun. The customer will be made aware that it is a lengthy process and the storage charges may be more than the gun is worth.
No I cannot sell somebody the required parts to lower the power and let them walk away with an unlicensed firearm on the promise that they adjust/repair it when they get home.
If it is possible for me to adjust the faulty gun to bring it within the law then that is a service which I would do, however you may have to come back at a later date to collect it when the work is finished and the gun is checked over the chronograph. You would obviously be liable to pay for any parts and labour required.
My advice to anyone is to buy yourself a chrono, learn how to use it and keep your guns at an acceptable power with a little bit of headroom just in case of the gun "bedding in" and the power rising. You don't have to spend a fortune on a calibrated chrono, there are plenty of cheap options that will be close enough to the accuracy of a calibrated chrono if you give yourself a little headroom.
Keep an eye all the guns you possess, a regular check prevents any embarrassing incidents.