Is it legal for a company to overwork their employees?

My fiance has a job at a warehouse that advertises as part-time to full time work and draws applicants in with its claim of having a regular schedule. My fiance now works 7 days a week on mandatory overtime. He is unable to plan a life outside of work because the few times he's had a day off they didn't tell him until the day of or before the said day off.
Workers are now overexhausted and I feel the company is putting workers at risk because workers are making more mistakes and operating heavy machinary and throwing heavy boxes on a day after day without time to recooperate. The business has no workers union. The people there have seen co-workers get fired when they have been caught asking other coworkers to "go out after work".
Is it legal for them to run their ship so horrendously? How can we get someone in there to do a little workers quality control?
They actually told my fiance he may not even get the time off for us to get married!
Please help.

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