You might have put your friend in a tough spot, but they should have put the sign where everyone could read it. If this goes to court, which it probably won't, you'd probably be fine. Still, they may be in a position to make the life of the friend you were visiting a bit more difficult, so you may want to give fair warning in case the idiots are vindictive.
If they do get vindictive, then go buy a few boots and see how they like it.
That'd be money better spent than paying a stupid fine.