I couldn't agree more...his partner should've been his first concern. Easy to say in hindsight but what he should've done is keep walking, get 1/2 way up the stairs and get the partner to call the police and get a safe distance away, then intervene. These fcukers are cowards and going by the vid have little or no fighting experience, sure there is a small chance that they will front but my money is on that they will run.
As others have said even if they didn't want to get involved a simple call to the cops would be enough.