Kind of a gray area here to be honest. Although you were not yet a couple you were potentially one, and she was very quick to head off to have casual sex with her "friend". I wish I had better advice for you but you have every reason to be concerned. Thing is her first impulse was to "fill the need" rather than be patient and wait to see what would come of you and her. She doesn't sound like she's a very commitment related person at all and I honestly worry whether she's the type (at this point anyway) to be marrying. What happens if you have to be apart again? What assurances do you have that she wouldn't do that under other circumstances? You don't at this point and if you consider a long term or permanent relationship with her I would wait for her to show you her level of commitment.

Give yourself a break a little here, it's normal to have the doubts and insecurities based on what you've been through. I wouldn't make a quick decision to get into anything long term with her yet. But that's just me.