Hmm. The owner of the billboard knew the message before he agreed to the business deal. So him being offended by the message doesn't seem plausible and if you are offended by that milquetoast message you aren't going to manage life well because it only gets much harder from there. Lastly, not everyone has the zoning ability or even placement opportunity to erect a billboard along a highly trafficked area.

My main argument is very simple. A deal is a deal. I hope this guy refunded the client at least a pro-rated refund.