​Sisters “business partner” claims zero dollars in income every year for taxes and is saying that it’s perfectly legal

My sister has this business partner/mentor who she’s working with and eventually wanting to merge businesses with due to her mentor retiring and wanting her to take over the business. She has been telling my sister to delete her quick books account and only receive checks into her account. She thinks that because it’s “cash” she doesn’t have to claim it as income. She pays all of her employees “under the table” but writes them all checks. My father wanted to buy the business when the merge happens and she told him that he would have to do it in all cash and gold bars. LOL

I don’t know if she genuinely thinks this is legal or if this is actually a way to get around paying taxes? Her revenue exceeds a million every year but she pays $200 in taxes because she claims zero in income. Supposedly this has been happening since 1997 lol. Can someone help me understand? Pretty certain it’s illegal but I know nothing about taxes and loopholes businesses might use to get around things like that. Am I missing something????