Anger is one of the emotional stages that we go through following diagnosis and before we reach acceptance. It's part of the "Why me?" self-questioning. :)

This isn't something I've necessarily come across as such, so it might be that something else is at play, or indirectly the MS could be affecting your friend. If for example, they find certain social situations quite difficult, their reaction to this might be aggressive if they don't know how to control or express those feelings. For me, I know that a late night social situation can be hard work because I'm usually pretty fatigued by the end of the day and so I tend to be pretty quiet, sometimes people can take this as me being grumpy or in a mood, the truth is my brain feels like mush and I can't get any sensible words out. Maybe try talking to your friend about how they feel and how they are dealing with things, I'm sure they'd appreciate it, after all it shows you care.