do you have enough good letters from more med school applicable letter writers?
I don't think you *have* to have an employer write a letter (but please double check I may be wrong) if you have say much better letters from say a doc you shadowed, a humanitarian type longterm volunteer gig, from science teachers, research mentor, etc
I saw that because the LOR writers I just spoke of, it seems to me, can speak better to your suitability as a future physician than a lifeguard
not that there aren't valuable and translatable skills from that specific occupation
but, for a future phyisician yes I want to know what sort of employee you are, but I also want to know what sort of humanitarian, scientist, scholar, researcher you are
if you have letters covered, it's fair to give her two weeks notice
don't give less, that is unprofessional
you will have to list this job and her as your reference, and if the med school did call her, they would ask if you gave appropriate notice potentially (they may not even call her!)
just lay down the law nicely and politely, that you are first and foremost a student trying to attend medical school, you have a pre-admission test you must do well on, and therefore you cannot do more than xyz. Let her know you let her know that previously. Now you are letting her know that if she asks for more than xyz than you will be forced to give 2 weeks notice which you would rather not do. This is a good test of your assertiveness skills. If she is a decent honest although stressed person, she will have to at least recognize that you let her know this previously and will either comply or not comply. All of this I say though, assuming you have enough letters lined up.
Lifeguard is not the type of job I would *expect* you to have a LOR for for med school if you have enough of the proper experiences also lined up.