I sort of see your problem that she is seeking to create problems for your friends who are working for you. And if the prime place of employment has issues with "moonlighting" there could be problems. I agree-- your STBX is shortsighted if you can get work done at convenient prices.
But she is entitled to do stupid and unkind and unwise things--it goes with the turf.
What exact issue is she pushing about friends on the home jobsite? Throwing mud does not make it illegal. But I'd be wary of moonlighting issues if I were one of your friends.....
I repeat my suggestion to check out federal postal laws about putting stuff in a postal box--if the person doing so is dumb enouh to admit it! (But I remind you not to get into mudfight with a pig!)