Conventional domestic systems almost invariably have a valve on each end of the radiator - an adjustable one at one end and a lockshield valve on the other. Nowadays the adjustable one is most often a TRV (Thermostatic radiator valve) at allows for automatic opening and closing of the valve according to temperature in the room.
However all of these types can be closed down such that the water is cut off both ends. In theory this allows one to undo the large nuts on the end of the radiator, drain the water out of the radiator and remove it without interfering with the rest of the water in the system.
Sometimes the reality is a little different and one of the valves leaks a bit.
But the concern I have about doing this to undertake the exercise you propose without reckoning on draining any of the rest of the system is rarely does a new larger radiator have exactly the same distance between the ends of the rad, so some minor adjustment that involves removal of the valve is often necessary. It might work or it might not.
You will have to rebalance the system using the lockshields have you change the rad in one part for a larger one.
"But the concern I have about doing this to undertake the exercise you propose without reckoning on draining any of the rest of the system is rarely does a new larger radiator have exactly the same distance between the ends of the rad"
If the new rad is exactly the same length as the old one then shutting of the valves both ends and removing the rad from the valves (leaving the valves on the pipe work) and then fitting the new rad in it's place is possible (although the water in the rad will still drain out when it's removed)
If the new rad is longer (or shorter) then the pipe work will need modifying, in which case you've not really got a chance of being able to do it without draining the system.
To be honest though, if your husband doesn't know this then I'd question his ability to do the job himself.
I guess that's the width, Funny?
One or both of the supply pipes are going to need lengthening.
I guess Mr Funny is going to use a freezing kit to avoid draining down. Either that, or he's going to fit adaptors to the rad tails.............. which would look awful.
I must admit, I froze some pipework recently to do this....... in a pub, where we didn't want to shut the system down for hours.