Parallette Training - Volume 1
the first half of the article goes through proper progressions of an L-sit. the ability to do this on floor also requires flexibility in the pike position and strength in the hip flexors and quads and hamstrings. if the hip flexors are weak/untrained, it doesn't happen. if the hamstrings are tight, you can't lift the legs at a 90 degree angle. if your abs are weak, you can't hold your legs at a 90 degree angle.
Parallette Training - Volume 2 - The Press to Handstand
towards the end of the article, it goes over "assistance exercises". these can be used to develop the active flexibility to hold the L-sit (pike) or straddle-L (straddle-sit/straddle lever)
A simple program is test her ability and what progression she can hold. If she can hold a tucked L-sit for 10s, she should try repetitions of 5s ( non-failure ).
For total volume, you could use something that Coach Sommer does being 60s of total volume. So if her max is 10s, she would try [email protected]
There are also other programs that involve splitting up the static hold work with working the movement through a RangeOfMotion ( dynamic/isokinetic ).