Boss Continues to Deny One Employee's Vacation Requests, While Allowing Others to Take Time Whenever

Most jobs legally require that full-time employees accrue paid time off and this employee wanted to cash theirs in. However, her boss constantly claims that other employees are either on vacation or denied her PTO. She works 40 hours a week, saying she's burnt out and is just trying to get some time off.

Advice in the comments tells her to go directly to HR, but the smartest suggestion is that she just look for a new job, because it sounds like this boss will never give her a vacation.