Have a student who has CFI-Helicopter, and looking to get CFI-Airplane. Student has his commercial single engine airplane and wants to get CFI Airplane. What endorsements and any other factors will she need? I am aware of what will be on the checkride, by looking in the CFI-A transition table in PTS.
Adding a Certificated Flight Instructor Category and Class rating to the holder of an existing Certificated Flight Instructor certificate.
In my opinion, the sample (or similar) Instructor endorsements (per AC 61-65H - Certification: Pilots and Flight and Ground Instructors, linked just below) documenting the training provided to and received by the student meet the necessary (endorsement) requirements asked in your question. This is based on someone who currently holds a CFI-Rotorcraft-Helicopter certificate and is adding a CFI-Airplane-Single-Engine rating. (This also assumes your student has an airplane instrument rating.)
The endorsements referenced by the list below are from AC 61-65H (beginning in Appendix A):
14 CFR Part 61.39: Prerequisites for practical tests: A.1 and A.2
14 CFR 61.185(a)(2): Flight instructor aeronautical knowledge test: A.42
14 CFR 61.187 (a): Flight instructor ground and flight proficiency/practical test: A.43
14 CFR 61.183 (i)(1) : Spin training: A.45
A 25 question written test per 14 CFR 61.185(a)(2), referenced in endorsement A.42 noted above, is required. Several other references show this requirement - see the FAA AIRMAN KNOWLEDGE TESTING MATRIX Effective April 24, 2023, page 10 . The knowledge test name is shown as "AFA" Flight Instructor Airplane (Added Rating)*.
If the flight test is a "retest" the examiner must test the person on stall awareness, spin entry, spins, and spin recovery instructional procedures in an airplane that is certificated for spins. This is opposed to having an endorsement, as shown in A.45 above, provided by the instructor.
You will have to meet all the requirements of §61.183 in order for said student to be eligible to take the practical test for CFI-Airplane, single engine. The chief difference for you, and for this student is going to be they will not be required to take the fundamentals of instruction exam again, as they already hold a CFI certificate with a rotorcraft helicopter category and class rating. They will be required to take an abbreviated version of the CFI knowledge exam specific to the aircraft category sought (see the FAA’s knowledge exam matrix). The transition table listed in the CFI-PTS should cover the pertinent areas of the practical test which the examiner will focus their emphasis on.