lol Rustic Elegance. I have never heard of that dress code! This must be from Pinterest. Sounds like you're not too excited about going. Just send a gift.
I'd go but if money was tight I'd forgo new clothes and possibly the hotel.
I'm fine skipping the hotel but I would have to buy a dress because I don't have one that fits. DH would need something as he doesn't even own a suit.
If you don't want to go don't, I just wouldn't let clothing keep me from celebrating and being with a friend. It doesn't sound formal so slacks and a tie could work just as well as a suit. Can you borrow a dress or alter one you own?
If it's DH's friend and he doesn't care enough to even make a choice, I'm not going out of my way for that.
This is where I fall. If DH wanted to go, I'd go. If he didn't or doesn't care, I don't know that I'd be worrying about buying clothes, etc, to go to a wedding that neither of us are super in to.
I had to go see where "rustic elegance" came from. According to this ezinearticles.com/?What-is-Rustic-Elegance-and-How-Do-I-Get-the-Look?&id=3843406, it's exactly what I expected--chalkboards, mason jars, lights in the trees....I don't think your H needs a suit for that! We went to a really pretty version of "rustic elegance" this summer and even the groomsmen didn't actually wear suits. So, IF you want to go, you could get away without the suit.
What does your husband normally wear to weddings or similar events? Whatever "rustic elegance" is, I'd imagine he'd be able to get away with his normal wedding attire. That sounds like it is on the casual side.
That said, it is okay to skip it if you're really not feeling it.