All people in my workplace try to make sure they don't use the same toilet cubicle as my friend with HIV. But I thought you could not get it from the seat.

HIV, the virus that causes AIDS is in blood, semen, vaginal fluids and breast milk. To create an infection, one of these four body fluids with HIV in it must come into immediate and direct contact with the bloodstream or a mucous membrane of another person. I include the word "immediate" because the virus cannot survive for long outside the body. Oxygen destroys the virus. If any one of these four body fluids were on a toilet seat, (a) oxygen would probably have destroyed it before anyone else approached it, and (b) a person sitting on a toilet seat does not expose the bloodstream or a mucous membrane to the fluid on the seat, so there is no potential mode of transmission. If you find a toilet seat with blood or another potentially infectious body fluid on it, make sure that the seat is properly cleaned before anyone else uses it for reasons of general hygiene, but do not be concerned about the possible transmission of HIV in that setting.