I'm pretty sure they can do this, especially with job shares. Main thing is to see what is in her contract of employment. Its not uncommon for local bosses to make up rules as they go along.
Usual practice (though not compulsory) is for both job share employees to get a pro-rata addition to their leave allowance to cover bank holidays, so a three day worker would get say an additional 5 days and a two day worker would get say 3 additional days holiday.
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If you're an employee and not a member of a union, its very hard not to be walked on whenever management choose.