Daughters friend age 15 kicked out, staying with us...?

She's a great kid but her dad (who I went to school with) is a fat, lazy weed smoking, daily lager drinking, jobless bully threw her out coz she didn't want to babysit her little brother 4 days a week while he sits on his fat backside and her mum does what he says. Now I've got that off my chest, she's been staying with us for a week now and I wanted to know if legally so long as her parents don't mind, which they don't, can she stay here permanent and can we claim tax credits for her ? Thanks. P.S. Her school are watching her coz he's bruised her a few times now.