I've done plenty of weddings for young couples that only want country. I can give you some tips and songs if you'd like. I have a feeling that this couple knows their family and friends and what they want. Sure you'll get a few people that want something not country but you can play with that.
Here's the key, there are several types of country. Do they want Bro-Country (Jason Aldean, Luke Bryan, Eric Church, Thomas Rhett),traditional country (Randy Travis, Garth Brooks, Joe Diffie), Red Dirt Country (Josh Abbott Band, Kacy Musgraves), something in between (like Zach Brown that dips into it). Do they want to two-step, country swing, line dancing?
Don't look at country just as one single style of music but as a genre that has different sub-genres and what people want. Trust your client.