My friend issued warning without investigation. Any legal options?

My friend works for a small company with no HR department. They were issued a formal warning after what was called a “fact-finding” meeting, but the meeting was run by the same person who accused them. The CEO even admitted it wasn’t a fair process and that “this is how the business operates.” No proper investigation was ever done.

They’re feeling cornered since their visa is also tied to this employer. What legal steps (if any) can they take to challenge this warning or protect themselves?