The guy has got 3 years over there.
The embuggeration factor alone of getting a FAA JAR piggy back would put me off. Never mind waiting months for paper work to wing its way forwards and backwards across the atlantic.
Get a FAA ticket and see what happens. Its not as if it stops him flying a G reg if he wants day VFR. If he then wants to continue on to do CPL etc his FAA ICAO ticket will do he doesn't need to swap it.
With a FAA he will be able to add an instrument ticket to it as well if he likes.
In fact the more I think about it he would be daft to go anywhere near a JAR training establishment and thats not including the fact that most of them are run by knobs.