I'm pretty sure the employer is within their rights to "force" work (within reason), however obviously if he took personal time for his absence then any time over 40 hours (during the makeup time) would be at overtime rates.
There would be a few limitations on the employers ability to do this - but probably not in your friend's favor. For instance if nobody ever works on a Sunday, he probably can't be forced to work on a Sunday. (But if that's part of the normal work schedule, then that time can be forced.) The idea is that the company needs to produce a product/service in order to be sustainable, and it relies on the employees to do this. It does also depend on the nature of the business and a few other factors as well.
Every state is slightly different, but in general your friend will probably have to make up the time if the employer forces him to.
