It could be anything or nothing. Some people are just rather cranky. It is scary to think about how people could snap and get violent, happens with and without mental illness involved. Studies I've read point mostly to drug/alcohol abuse being a big factor in bp violence, because bp people are not traditionally inherently violent.

To help your friend, she would be at some risk of bp since her grandma had the dx. I think it's nice to be there to talk, encourage exercise, good nutrition. Hope that might be of some help.