Contractor using green board for shower

[Mike Girard]

My contractor's crew began tiling the bath/shower surround today. It was such a mess I pulled it all off before the mortar could dry resolving to at least get it properly started myself. In researching tiling, I learned that it is considered bad practice to use green board for shower and tub tiling Green board is what my contractor used. Now that it's all up, I don't want to have a confrontation about ripping it down and starting again, and I'm unwilling to do it myself. 

Three questions: One, Is green board even allowed for shower/tub surrounds in the Philadelphia Building Code? Many municipalities have forbidden it. 

Two, is there anything I can do to make the green board work, such as applying Redguard? I've seen a lot of people recommend Kerdi, but I don't have the budget for that. 

Three, is this an error so bad that I should not work with this contractor on future projects?