The short answer to your question is Yes what you described is symptomatic of MS.
Impulsiveness (i.e. over talking, talking out of turn, tangential thinking) are all symptoms. Getting overly frustrated (i.e. bitching as you label it) is also a classic symptom and not something an MS patient can just turn off.
With a healthy person I would have no issue counseling behavioral shortcomings. With an MS patient with brain lesions that affect behavior its a different story. If you are a good friend try to reduce the stress, be supportive and be there to help in non judgmental ways.
This may be tough advice but you should take it to heart. You can play a positive role if you so desire. If not, I would avoid being the person that adds stress to the person because the disease negatively impacts their behavior.
Take it or leave it.