There's a subtle but, IMO, significant difference between:
Am I the only one who thinks that this is a really poor answer?
Hey everybody, downvote this answer!
Basically, yes, it's OK to discuss specific answers in chat (or elsewhere) and to ask others to take a look at them, which, if the answer is indeed bad, might lead to it getting downvoted. (However, one should be careful not to violate the "Be nice!" policy while doing so — in particular, try to stay polite and objective, and focus on the answer itself, not on its author.)
On the other hand, asking other people to downvote an answer just because you say so (or because they're your friends, or for any reason other than their personal evaluation of the answer's quality) is, at best, poor etiquette. At worst, it might be considered abuse of the site (e.g. "intentionally abusing the system to manipulate someone else's reputation", to quote the help page on serial voting), and it's possible that moderators might decide to intervene if they find you doing it blatantly enough.
Of course, in practice there's a wide gray area between these two extremes, and, without the ability to read minds, it's not always easy or even possible to tell if a particular comment is intended as a legitimate request for independent evaluation or as abusive voting manipulation.
The general rule of thumb, however, is still clear enough:
- Asking people to look at an answer and vote it up/down depending on what they think: OK.
- Asking people to downvote an answer if they agree with you that [insert legitimate reason to downvote here]: probably OK.
- Asking people to downvote an answer just because you say so: don't do that.
In any case, if an answer is truly bad, you won't really need to tell other people that it's bad. It's usually quite sufficient to just draw their attention to it, and let them draw their own conclusions. (See also: What is the "meta effect"?)