You can get vegan cheese replacement and I'm wondering if she told the customer if it was vegan friendly?
Years ago I was out for Xmas dinner with work and another member of staff and I ordered a vegetarian meal. We both found a chunk of meat hidden in the middle of it. She was a boss and I was glad I wasn't on the receiving end of her temper that night. 😂
I used to go to a very nice restaurant with my husband when he was receiving chemo, as it was on the way home from hospital. He would have a fry and I'd order beans on toast. The beans came in a wee pot and when I got to the bottom there was a small piece of bacon right at the bottom, in the centre. The first time it happened I thought it was an accident, but found it odd as my dh didn't eat bacon, so there wasn't any on his plate. Same day, next week, EXACTLY the same thing happened. I didn't go apeshit but explained to the owner that in no way would I accept that this was an accident two weeks in a row. I could hear her yelling at the chef and the waitress, who was lovely, confirmed that he was a wanker, so she was delighted.
I don't trust chefs as some of them seem to have an irrational hatred of vegans. Thankfully there are now some wonderful vegan/vegetarian restaurants now.