From what you posted, it would probably be best for your friends to park off site/in the street instead of on the Landlord's property.  If your friend's car has been towed, the issue is between your friend and the tow company.  If the tow company broke the law, then your friend could attempt to get a refund of the towing fees/costs from the towing company, right?  In other words, your knowing the person whose car was towed does not make the issue a Landlord/Tenant issue.

You mentioned in your post, "...we received a parking decal & were told you had to have this decal on your car or you would be towed...".  It does not sound like you received anything else from the Landlord re: legal or allowed guest parking.

I can certainly understand your frustration with the lack of visitor parking at the place you rent.  It would be an inconvenience.  Unless you can find a local law or code that makes your Landlord responsible for visitor parking, however, you will probably just have to wait until your Lease expires, give proper move-out notification and then move to a property that does offer reasonable guest parking.