It is not really a `gift; if the son gives Mum the money to buy the gun. If you bought a diamond bracelet for your wife/girlfriend/mistress and gave it to her then I reckon she would be very happy.Don't think he meant the mum would "sell" it to her son, but gift it to him.
If you then said "you owe me £1k for that" then maybe a different response as it is no longer a `gift'.
If the Mum uses the son's money to but him a gun then she is not `gifting' it to him. In practice, they would get away with it. The same would apply if you had a friend taking up airsoft and he gave you the money to by a RIF for him under your UKARA number. What you could do is buy the RIF and loan it to your friend until he obtains his defence and then sell it to him In the meantime he may give you a security deposit (equal to the cost of the gun) to cover any damage etc.
Lots of ways to get round the regulations but not strictly legal.