Notice Period

My sister got a job at a new company. her previous employer has a notice period of one month but without the option to pay in Lue, supposedly. She wants an early release since the new employer wants her to join sooner than a month. also unfortunately she is undergoing a back problem due to which she is forced to take some bed rest. The previous employer are forcing her to, take some rest if need be, but come back and serve her one month notice period with no concession at all. Hey are her options ? Does she have to serve the one month period even if it may prolong her joining her new employers ?