This is a hypothetical thankfully but I was listening to a podcast here in Ireland and a woman wanted to report her friend who is a nurse because she admitted to being constantly high on cocaine and speed even before going to work.
A few people were saying it was none of her business but many others said that it stops being about her when she goes to work under the influence.
I know drug testing in jobs isn't common in Ireland/UK like it is in the States, but have thought that for high stakes jobs (i.e. construction, medicine, aviation), employees would regularly get tested.