Article is here.

In case some of the newer guys don't know, pre-selection is a great example of the groupthink women exhibit. Since women are all about conformity, if everybody else likes X, then an individual women will like X as well. This is how name brands form, how fashion makes things go out of style. Advertisers simply scream "everybody else loves X, you should too!" and they believe it.

When applied to men, the same goes. Once you're taken, all the other girls will recognize you as someone worth being in a relationship with, since this other girl approves of you.

Edit: as /u/RPsixthree pointed out, this is just another form of social proof.

OP of that post: "some of her friends have been super friendly, always saying hi and wanting to chat, every time I run into them."

On a side note, the fact that her friends are now trying to swoop him isn't that surprising. After all, honor is a male abstraction.