Is it legal to forbid employees from speaking to laid off workers?

it is legal to ask. or even instruct you to do almost anything

if you fail to do as asked you must face the consequences of your actions............

if they fired you would they be able to show they had "cause" to fire you and therefore cause you to be denied unemployment?

probably not but the job would still be lost and the termination would be legal just without cause.....

Legal or not, it's just never a good idea to speak to former employees. It results in dangerous gossip, backstabbing and it could cost you your job. Laid off employees can be disgruntled people who want to vent their ill feelings toward anyone who will listen. Don't take part in that.

There is no reason to do it. They were laid off, you have your job, just be content with that. It may be insulting, and it may be illegal, but you don't really want to find out first hand.

No, it isn't legal on too many levels to count. That manager has set up the basis of a very expensive lawsuit if she follows through on her threat.