Most workers have the right not to work more than 48 hours per week (averaged out over 17 weeks), however there's an exemption to that rule in jobs where 24-hour staffing is required. So that part of the relevant legislation might not help your friend.
However ALL workers have the right to EITHER one day off per week OR two consecutive days off per fortnight. (A 'day' here means an uninterrupted 24 hour period). They also have the right to 11 hours rest between the end of one working day and the start of another:
https://www.gov.uk/rest-breaks-work
So it's possibly those rules that your friend should be reminding her employer about.
The staff on the Acas helpline may be able to assist. The number is 0300 123 1100, with lines open weekdays from 0800 to 1800. (Calls are charged at the same rate as those to 01 or 02 numbers, from both landlines and mobiles. That means that, if you've got spare 'included minutes' on your mobile phone's 'bundle, they'll be free). Several AB members have reported quite long waits to get through to someone on that number but that it's always been worth the wait.