Come on Airborne Artist, you have been around long enough to know the answer to your own question. Of course you can transfer between Services and there are formailised instructions on how 'candidates' (or your friend
)can do this. Just ask your friendly desk officer, poster or appointer. They will undoubtedly have all the information to hand and guide you/the individual accordingly.
As to the easiest Service it will always depend on supply and demand - just as any other company/HR management. At the moment there is a pilot shortage that is forecast for a number of years with a current sensible QSP bridging package to 'grease the wheels' into civvie strasse. But in 5-10 years time you could be Gods gift to aviation and all of the Services may be full with not a hope in moving between Services if the airline/civilian sector changes (and we all know that it is cyclical - currently on the international rise for the next 10 to 15 years if you believe Flight Interntional).
What is truly in favour these days and now widely accepted is Jointery and with JFH and JHC then mixing Services has potential to add Joint experience above and beyond the usual exchange programmes - both in the cockpit and in the necessary desk/operational staff tours.
However, based purely on what goes on PPrune my gut feeling is that the Teeney Weenies would be the easiest to transfer to as not only are they are also involved in the global shortage of pilots that is seeing their pilots easily exit into civilian flying jobs as per the other 2 Services - but the whole Apache spares/support/flying rate debacle combined with the very uncertain future of FLynx is resulting in a very uncertain future for the Hairy Arm Corps and therefore low morale is and uncertain future is adding to the 'push' factors.
But - ask around, go to the horses mouth, visit these people and find out for yourself - then go to the manning/personnel desk to gain advice on how to effect a transfer.