Hello, I'd like to help you with your question.
Being passive aggressive is a defense mechanism which is learned in order to cope with situations where the person is unable to express anger. It is a way to get that anger out without being direct. Another possibility is that your friend has a personality disorder. Both issues can be caused by unresolved abuse or neglect
from childhood where the person did
not get their emotional needs met. So they developed alternate ways of getting what they need emotionally, which fits the situation they are in but is usually maladaptive otherwise.
Usually someone with defenses or a personality disorder such as your friend has is difficult to treat. The person first has to realize they have a problem and it's not everyone else's fault for how they feel. Most of the time, due to the nature of the disorder, they do not have insight. So they are unlikely to seek help, leaving them to continue as they are already.
One of the best ways to deal with someone like your friend is to try to eliminate them from your life. They are not going to change, no matter how hard you try to help them or be there for them. So you will continue to have the same relationship regardless of your effort. To remove them from your life, stop having any face to face or phone contact. If they call you, don't return the call. Avoid them as much as you can. If they ask you why, tell them you are busy or another legitimate excuse. Take them off your Facebook, remove their numbers from your phone. After a while, they should move on.
If you cannot avoid them completely, try reducing your contact as much as possible. Avoid contact unless you must be in the same place. When they talk with you, offer short, neutral responses. If they try to rope you into an emotional situation, have a neutral response ready like, "Sorry you feel that way" no matter what they say to you. Repeat as needed until they lose interest and leave.
The only way to cope with someone like your friend is to reduce contact as much as you can. Unfortunately, everyone at one time or another must deal with people who act out. It helps to seek out others who are not toxic and develop good relationships so you can have a balance in your life.
I hope this has helped you,