Excusable reason for late to work?

Why wouldn't you 'risk' it? A persons life is more important than being late. Whether or not an employer punishes you for this is simply up to them because they make the rules. If they're a heartless moron and do decide to punish you for it, oh well. You did something awesome by saving someone and at least have that to think about. Knowing you're a good person and are willing to make a sacrifice, even something as menial as being late, to save someone else's life is far more valuable, respected, commendable, and worthwhile than a garbage job with a garbage employer, IMO. If you had gotten fired for this, I'm sure your next employer would be inclined to agree.

The employer doesn't have to accept your reason but who cares right?  You've done your human duty regardless of what your boss says or does.  Even if you got fired that's less important than saving someone's life

Of course they can.  What you do in your personal time is not their concern.  Is it ethical?  Of course not.  If there are defined corporate rules though, then you may have to go to a higher level in order to have an exception made.