If an employee doesn’t want to come back to work after maternity leave, she will need to provide you with her normal amount of notice, in the normal way.
She will continue to build up holiday entitlement until her maternity leave or notice period ends (whichever is sooner).
She won’t have to pay back any statutory maternity pay she’s received, but if you’ve provided her with enhanced benefits, she may need to return a proportion of any payments to you (depending on her contact with you).