Snatching, so long as it's in the term I understand; that is to say, she breaks it off before you start to date, is fine. She has the freedom to end a relationship if she wants to.
If it's cheating, however, I'm not so hot with that.
She left him because she didn't want to be with him more than she wanted to be with you. Pretty sure she has a right to make that decision.
As marriage is a pretty formal, legal declaration of unity, screwing around with a marriage is a little more serious. Just like any civil partnership, or a long-term relationship for those who do not wish to, or are unable to get married.
But similar rules still apply. If someone wants out, they have the right to make that decision. It's a more serious consideration though. But so long as it's over before the new relationship starts, then it's fine. So long as it's not cheating.
And to the main original point, I don't think I'd be all that paranoid. We've got the right to change our minds in life, and you have to accept the hand you're dealt when it comes.