Not that I can help with removal, but if required I have a brand new pair of a Targa 52 2008, might be the same if he can get the old ones out to measure them or find a part number, let me know if this is any help.
Our Targa 52 with D12 engines did exactly the same, we just left them at the bottom of the filler tube and they never overflowed again, they take a lot of coolant so wouldn't have thought a litre or so would make any difference, but just check regularly until you are confident they are not...
QUOTE. What if he forgets to open the sea cocks?
We always loop the engine keys round the sea cocks after nearly forgetting to open them one day, has worked for us for the last 15 years