Ultimately, there’s no changing that someone very near and dear to you is starting a new decade of their life, and big elaborate parties aren’t for everyone. You know your best friend better than anyone else in the world, so you know if that’s something they’ll enjoy. But if it isn’t, that doesn’t mean there’s nothing you can do to make it special. 
Take them out for breakfast or dinner at their favorite place to eat, help them catch up on housework they may not have had time for, and if you’re planning far enough ahead, write them a bundle of “Read Me When…” letters for them to open when they’re sad or when they’re happen or feeling lost, or just whenever. You can buy pre-made bundles here that you just need to fill in, or you can make your own at home.
Since it’s their 40th, 40 letters would be ideal, but that might be unmanageable. Though if you can’t manage letters, a bundle of $1 lotto tickets or scratch-offs could also do wonders and would make an interesting investment should they be winners. A useful distraction when cooking breakfast.