my friend is having resignation problem below:
Based on her current employment contract in company A, she needs to serve 2 months of notice period or payment in lieu of notice.
Now, she got an offer from company B to buy her out for 1 month. Thus, she tendered her resignation for 1 month only and requested to buy out another month. However, the company A does not accept that. Instead, company A wants her to serve full 2 months before she can leave.

What can she do if she has already signed the offer from company B?