I am not aware of any training material about this since it should be common sense that there are always going to be random splashes and risk of spillage - accidents do happen.
However, if your partner is happy to continue not to use dust sheets then the best thing to do is not interfere, she is bound to have a mishap at some stage and will learn the hard way.
This video sprang to mind, btw
https://twitter.com/i/status/1643111538486829056