Love vs. principles

A friend's in love with a married man. She seems a magnet for this type. Then again, it's probably due to our age. Being in the mid-30s, the choices of guys to date are fairly limited. She's not the sort to date younger guys, so she goes for guys our age. Most of the time, these guys are already married or involved in long-term relationships already.

The first time a married guy was interested in her, she was extremely into the guy, but she backed off. The guy was willing to leave his wife and children for her, but she told him no. That his priority should be his family. The second time is with this current guy. They're together, although in a long-distance relationship. She doesn't really like to talk about their relationship, which leaves us, her friends, to rely on pure guesswork. All she'll tell us is "yeah, we're doing ok." Or "I'm going for a weekend trip to meet with him."

Of course, I'm not one to scold. We're old enough to know what's right and wrong for us, and what serve our best interests in the long run. What mystifies me about the whole thing is why, on the first round, she fought for her principles even if she really cared for the guy. In the second round, she went for it completely.

It got me wondering, when does principles win out? Up to what point do you hold on to your principles? And of course, when do you let go of your principles, and follow your heart instead?