chicken in buttermilk.......my recipe calls for chicken to be soaked in buttermilk over night.

I've done 3 nights before and it was very good. Some recipes even call for such long times: https://boingboing.net/2015/02/11/how-to-make-the-absolute-best.html

I've done this. The chicken's going to be fine, even fried.

Your chicken is still safe, providing it's been in the refrigerator. However, it's going to be kind of mushy when cooked. Buttermilk is acidic, which will have been breaking down the fibers of the meat. It's used to tenderize meat, as well as to help it hold liquid. I'd go ahead and cook it, but be prepared for more of a chicken mush than usual. Maybe chicken salad material?? I don't think you will care for it fried.

has it been refrigerated? If so I don't think it needs tossing from a point of view of safety. The texture however may be affected.