Couple of things here bugging me. First of all to all the sanctimonious folks wanging on with their astonishment that some people like bags... jeez, get over it. There are days when I get more enjoyment from my lovely little Mulberry collection than I do from my boyfriend! That doesn't make me shallow.
Secondly, this idea that hard work=riches and poor choices=poor income. Let's disabuse ourselves of the notion that effort and reward are always relative- nurses work a damn shit sight harder than footballers do but it'd take a nurse a decade to earn what some footballers make in a week (OP I'm not suggesting for a moment that you don't actually work hard or deserve every penny that you earn).
I can see both sides here. When you have something lovely and new you want to have it with you, so you can give it occasional loving glances and feel a little bit more special and have a little extra spring in their step. I'm not gonna judge the OP for enjoying her bag. So yes, she took it to work with her. Hardly a crime.
If this outburst of the less fortunate friend's is out of character then that to me would suggest that she is under pressures at the minute that maybe she hasn't shared with the OP and their other friend. I've been in the situation where I seemed to always be the one out of my friends who was struggling or facing some difficulty or other, and I just stopped telling them about it because I just didn't want to be that friend with all the problems. I congratulated and celebrated with them when they got promoted, I ooohed and ahhhed over whatever lovely thing they'd treated themselves to, and I tried to pretend to them that my life wasn't just one fucking nightmare after another. Maybe that's what she's been doing.
In this case it seems that the sight of the beautiful Birkin which represents the kind of lifestyle that she will probably never have was just the straw that broke the camels back, and the OPs friend lashed out. Yes it was wrong of her to do this, but envy and resentment are very human traits that every single poster on this thread will have felt at some time or another, so let's not judge. The personal comments that she made about the OP and her DH were unfair and unkind, but from what we've learned of their friendship it sounds as though she was just lashing out and didn't actually mean it- it was coming from a place of unhappiness at her own situation and she was just projecting it onto the OP.
The thing is though OP, you responded in kind. Instead of just walking away after her rant, which would have allowed you both the time to cool off and her to come to her senses and apologise, you retaliated with some things which true or not will have been very hurtful to her, and it sounds like she's already in a bit of a vulnerable state. That's why, in my opinion, you should apologise first. I know you didn't start the whole thing, but you delivered the knockout punch. Talk to her, and see if she has any other problems going on that perhaps she hadn't told you about. And, once you've got the bottom of it and cleared the air, move on.
Oh, and Elke- you need to tell me the magic trick for not paying full price for a Birkin!!