If your friend takes a spot on another trip leader's (TL's) permit, then your friend will have to travel with and camp with that group. The only way you can transfer a spot from one permit to another is to have another TL, with more capacity than they need, call in and decrease their group size. This allows that permit spot to go back into the general pool, and then you call in and add that spot to your permit. It's not guaranteed to work as someone else may pick up that spot before you.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse