Fit notes/return to work

I'm hoping though not exactly about UC directly someone may have useful info.

Younger person has survived life threatening cancer after a year and a half's gruelling treatments. Still has bad fatigue and various lasting aches and pains caused by both the cancer and the treatments. Helpful GP has provided a fit note for phased return to a demanding, shift-work job [catering]. However after a short time the employer is rostering the person as if nothing ever happened, 12 hour shifts back-to-back etc. Please does the fit note have any actual force, if so, how can they get the employer to be more reasonable and eg limit hours, and days worked?

Many thanks in hopes to B&W Mods and Community

Thanks Gary, yes, supposedly they are on a phased return. I hadn't thought of ACAS, I had suggested joining a Union though it's a very un-union area of work. Very grateful for your reply which I will pass on.
Denby