Nail biters are awkward to do so that the nails last, but they can be done.
I think L/P is a better option as it is so tough but you can use gel successfully if the client plays along and takes care of her enhancements and you do not make them long.
Of course the nails are not as pretty as when done on a more normal nail but for a biter who has had to look at ugly nails for years, they will look beautiful to her.
I find using nail enamel is the best way to 'make over' a biters nails when they are out of proportion. Nothing worse or more ugly than long whites on a short nail bed. Enamel also acts as a trigger to remind the biter not to put her enhancements in her mouth and it helps them to stop picking.
You just apply the tip to what is there and remove enough of the well first to expose at least half of the nail plate. Then apply your product.