Country club wedding outfit

So my friend of many years is getting married. Until recently I never saw this side of her. Her wedding is at an upscale country club.

What would be appropriate to wear? Im thinking a long darker colored dress.

But what about my husband? Would a polo and khakis be okay?

We are not dressy people so this is all new to us and I dont know anyone to go to for ideas.

Thanks for any help!