It is good to accept and give her the relieving and also settle her accounts if you feel the matter is genuine. It is only humane to consider such cases and there need not be any rule if it is in the interests of the employee. Moreover night shift is not an easy work schedule especially for the women. Medical issues are bound to arise due to biological changes in the body and even many men are getting affected due to it.
You may ask her to hand over by coming during day shift [if that is required and viable] so that the transition is smooth and person taking over has no issues to start on his/her own.
Thanks and Regards
However the management has option to get harsh and ask the employee to buyout the two month notice, but it appears a willful harassment by management to other employees, balance if the employee has submitted an acceptable medical document, then management has to have a lenient view always, and release the employee, however if the management has some pending assignment / works with that employee he may ask the employee to finish the same asap , or make it transferred to some other employee with full details.
Getting always harsh is not in favor of employer, unless any incident happens towards breach of code of conduct.
A healthy working atmosphere can be generated with co-ordination between employee and employer only, otherwise it would generate chaos and will effect the ambiance of workplace