Can a third party alter an artist's design without the artist's permission?

A friend of mine designs posters for local theatre companies. He does this work frequently pro bono for friends, but he also gets paid for it. He is also widely known to be one of the best designers in our city. Recently a theatre he designed a poster for (the "guest" theatre) turned his design over to another theatre (the "host" theatre), from whom they are renting space for a performance. The proprietor of the guest theatre signed a contract allowing the host to alter any materials submitted to them by the guest, after which the host made substantial changes to his design, including removing his name, without contacting him to obtain his consent. My friend never signed any contract with the host theatre and has always been very explicit with anyone he designs for, including the guest theatre in this instance, that his work is never to be altered by a third party without his express written consent. Does he have any legal recourse against the host theatre in this instance? They were given all of his contact information and he never found out about any of it until one of the actresses in the production posted the altered artwork, minus his name, on Facebook. They host theatre is actually something of a bully in our theatre community, and I can't imagine that this aspect of their contract, which all guest theatres must sign, is legal. He's more than willing to sue if he has a leg to stand on, but doesn't want to spend $300-$500 for a consultation with a lawyer if all the lawyer is going to say is, "Tough nuts." Thanks all!