something neat casual, like something you could also wear to a beach wedding, or a big family bbq.

Avoid stilettos or open toes, as you may be walking in the paddocks, or over gravel/sandy roads on the property, nice sandals are ok, but stilettos will sink, and if you are in the paddocks at all, you don't want toes exposed to grass spiders or snakes.

Boots, knee length skirts, all good. Your man should be ok with jeans and shirt, as long as he's avoiding the cliche cowboy style, and it's a neat tidy pair of jeans, nice shoes too, not blundstones lol!

Google image search on farm/ranch weddings for examples maybe?