being forced to take redundancy or zero hour contract

[Bossspud]

Hi legal beagles, I am an avid reader of the forums and would very greatly appreciate some advice on behalf of my sister
My sister has worked for a company for the last 10 years, 16 hrs a week, she and her friend were called into a meeting today and was informed that she could either accept redundancy or move to zero hours contract and she has to let them know by tomorrow her decision, my sister and her friend are only a few of the staff currently on contracts and it seems the company wants just to have people on zero hour contracts, I have informed her to tell her manager that she wants everything in writing and that she needs time to get some legal advice, however within an hour of finishing work she and her friend have been sent a text message reminding them they need to respond with a decision by tomorrow, I think this is an appalling way to treat loyal staff and trying to enforce unfavourable terms with their employees, please can someone advise on how my sister can move forward, is this legal, she and her friend are very distressed
Thanks in advance for your advice
Bossspud