I'm with Thunder Bow ... Intervening could very well put you in harms way.

The best thing you can do is remove yourself from the situation, and once you've established your own safety, then and only then explain to your friend why you left. Tell her that if she wants to see you in the future it will have to be in a "safe" (public) location without the hostile bully anywhere around.

Tread careful, dear ... These things can turn deadly right quickly! ... Remove yourself from the situation before you say anything, your very life could depend on it!

If there's a local counseling or domestic violence service nearby, contact them for advice on how to safely extricate yourself from this!

Don't rule out law enforcement too if necessary!

Sincerely,
Pfrog!