It is necessary for a Mexican friend who is awaiting her legal residence status to fly to another part of the U.S. She does not want to jepordize her status but needs to make this trip. Can her lawyer give a letter of permission to fly to where she needs to go (for only 5 days)? Thank you.

Once she's in the US (legally) she can fly anywhere in the US. You don't pass through immigration/customs officers on domestic flights. That's what it sounds like you were asking