Weddingbee boards

Hey Bees, I need some help with this:

My friend is having a very casual backyard wedding ceremony on her parents farm.  We think that most of her guests will understand that they are not expected to get extremely dressed up, and what shoes are appropriate for a wedding where everyone will be walking around in the grass.  

However, her fiance is from Cypress and is expecting to have a good group of people attending the wedding from overseas.  Her understanding is that their culture's wedding etiquette is to wear tuxes and very formal dresses to weddings and that guests who attend the wedding dressed that way may be upset if their shoes and other clothing get a tad dirty.  We are unsure what is the best way to let everyone know what the best attire for the wedding is though.  Should she do it in the invitations, and how would you word that?  Any other ideas?

I would say word of mouth or via their website (if they have one). Not everyone is going to know that they will be walking in the grass. Have the couple spread the word through family and friends.