Hi. Unfortunately not unless its in her contract that she can change hours. The company may have employed somebody else to cover the hours that she dropped when she returned from maternity leave & there may not be further hours available.
She can certainly put in a flexible working request to increase her hours but it will be up to the company to decide if they need any further hours from your friend, although if they decline she may be able to request an increase as soon as somebody else leaves or reduces hours, although again they don't have to agree.
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