1) It's doubtful that Dowd plagiarized. She's too experienced to think she could get away with copying such a well-read, online source.

2) It's even more doubtful that a friend conveyed the paragraph over the phone. It's too long and precise.

3) It's conceivable the friend e-mailed the line, cutting and pasting it, inadvertently passing it off as the friend's own language. So why wouldn't Dowd just say that? Because then there would have been an e-mail record for the Times to check. If there was no e-mail, Dowd couldn't use that excuse and get away with it.

4) That she didn't cite anything in the column suggest the line came from another person working with her at the Times. That person may have committed plagiarism, perhaps by accident as well, and Dowd may be protecting him or her.

This one occured to me as very likely also.

An assistant would come in handy doing a regular column, and one of them inserting a paragraph they found online is far more likely than anything else here. most especially the utterly absurd "conversation with a friend" explanation which was ridiculous. No one speaks in perfect copies of things online, let alone her then remembering it verbatim later.

She may be protecting more than the assistant, having people know that she doesn't actually write the columns herself may be something she's more afraid of than we realize. I would think it's extremely likely, but maybe she thinks most people would find it shocking to know.