used that restroom since it is for employees only. Both of those things can be true...but my guess is that vendors and visitors of employees often use the employee restroom. Even assuming the vendor breached protocol by using the employee restroom, I would say it is a pretty minor thing and so she is still 95% at fault and he is 5% at fault. If vendors using employees restrooms is normal, then she is 99.9% at fault and he is maybe 0.1% at fault (for single toilet bathrooms, I usually knock before opening a closed door).