Joe,
It takes a few steps.
If you go back to 61.63.b.1 for additional category ratings:
"Must have received the required training and possess the aeronautical experience prescribed by this part that applies to the pilot certificate for the the aircraft category and, if applicable, class rating sought"
This essentially means that although she has a Private certificate with rotorcraft rating, she is viewed as a student pilot with respect to the airplane category rating and therefore you have to comply with 61.87.L.1&2 endorsement since it is part of the required training for a Private Pilot, Airplane, Single-Engine Land certificate and category and class rating. What should be rolling around in your head right now is to make sure you cover all the requirements in 61.87,89,93 and 95. She might fly through them but you still have to hit them.
Clear as mud?
A