Ethical answer:

No way. This is stealing. You know that it's wrong. If you were honestly wondering whether or not it was, you wouldn't have been concerned about posting anonymously, so you answered your own question already. Try to get the miles the honest way - by offering an exchange, guilt-tripping them, or annoying them about it until they give them to you to shut you up. The fact that you have their account info leads me to believe that you are or were related to said mielage waster. If so, that'll probably come back to bite you in the a$$ at some point.

Pragmatic answer in case you opt for the miles-stealing anyway:

OK - so lets say you have a really good reason to hate this person and are just out to get them - I'm not sanctioning it, but we've all probably opted for petty vigilantism at some point or another. Objectively, I don't think that it is pragmatic to grab the miles. To use them on an award for yourself, you would be entering your name and personal information into the records at some point. That means that if they ever did find out, they can trace it back to you. So ethics aside, I think you'd get caught at some point. (Unless of course there is someone else you also hate and you get the ticket in their name unbeknowst to the first person and then tell the person whose miles they were that the second person you hate stole them and thereby pit them against eachother. They can't trace this back to you since you had the foresight to book online from a internet cafe PC where they can't trace the IP or login back to you. Then that's workable. But still really wrong. Beautifully wrong.)

peace,
~Ben~

<<<Disclaimer: This opinions expressed in this message are my own. My scruples stepped out of the room after the first paragraph and my Jiminy Cricket does not approve of or acknowledge the contents herein. I do not encourage others to bend or break the law since nobody ever wants to get sued or sent to the pokey. No warranties expressed or implied. Do not taunt Happy Fun Ball. Happy Fun Ball may cause vertigo.>>>