If he wants to change tunings, he will need to have it set up, professionally, because with the lowering of tension like that, you'll probably need a few more/less springs in the cavity to compensate for the tuning.
It would be better to have a second guitar, because with a Floyd, you can't just switch from E to D, without having to do a whole set-up.
Read about Floyds and you'll understand. They are based on tension. Even changing types of strings can throw off your tremolo.
Sometimes I hit notes only dogs can hear.