Brooke Shields went through Casa Bonita in Denver this week.
She used a alias name to gain entry to the restaurant.
Shields insisted to approach Trey Parker and Matt Stone, the owners.
She wanted to discuss reports from staff about unsafe situations and inadequate pay.
Workers also cited health risks.
Employees say they encountered backlash after talking about problems.
Following her visit, Shields shared solidarity for the staff online.
She posted links to their statements about the restaurant.