I don't do criminal law, but so far as I remember from law school....
The right to remain silent does not attach until you've been placed under arrest. If you are a witness in the investigation of some other person, I suppose you could refuse to cooperate without any legal consequences, though I think it could also be construed as obstruction in some circumstances. I can't say what those are though.
I cannot imagine a lawyer counseling a witness to a crime, not a suspect, to "keep your mouth shut" Again - if you're not being accused, there aren't that many legal rights attached to you at that moment. Oh, sure, you could chose to be a dick and just not help the cops, but I have a hard time thinking a lawyer needs to be involved for that advice, and I'd be surprised if a lawyer offered a client that kind of advice.
And... in most circumstances... witnesses don't have - and dont NEED - lawyers.