Guest attire at wedding?

All you really need to know is the time of day and whether it's inside or out. It sounds like it's during the day and outside, so it will probably be warm. You can wear a dressy sundress with heels and some nice jewelry. Be sure to bring something for later in case it gets cool. Weddings are rarely casual no matter the setting. Farmhouses and rustic settings are popular of late.

I attended a similar wedding in the fall, and people were still pretty dressed up, but not in evening gowns or anything. A cocktail dress would do if it's later in the evening.

P.S.- considering the fact that your friend doesn't seem to know much about the wedding, ask to see the invitation. He may have just assumed he could bring a date. You would be surprised how common it is for people to be this clueless. You don't want to walk in and end up having the hosts awkwardly scramble to find a seat and meal for you. It would be embarrassing for everyone involved.

Edit: Both dresses are appropriate. I slightly prefer the first one.

Can you ask him to call his sister or brother (whoever the nephews parents are) and ask what type of event it will be? Both dresses are pretty and can be casual or more formal, I don't think you would stand out either way. I prefer the second one in case you want to take a vote lol, but the first one is quite nice as well.

I am having my wedding next month and it is also outside in a garden during the day. I didn't specify dress code although I believe most women would have a comfortable dress on, nit formal, just a sundered or a longer floral print. It will be early next month in Ny so it ll be in the 70s. I would just want them to be comfortable.

Those would both be quite appropriate.

Do make sure he checks to see that he gets a "plus one" on his invitation-

and if it is true that it's to be an outdoor wedding, find some nice shoes that don't have stilletto heels - you don't want to be yanking them out of dirt if it's on grass. Anything with a chunkier heel would be fine.

http://www1.macys.com/shop/product/anne-klein-shoe...

I much prefer the second one. It looks a lot dressier without being too dressy. And I agree about the type of shoes that someone suggested. Much easier to walk on grass in something like that.

I love the first dress, but either dress would be appropriate for the vast majority of weddings. They aren't too formal nor too casual.

then he needs to call his sister back and ask what you and he should wear. Does he need a suit and tie? of khakis and a polo shirt.