I have one friend that I can no longer send texts to, as of a week ago. That friend can still send mesages to me and has not blocked my number. I have turnerd the phone off/on without effect. I tried deleting the text number. I deleted our threads. I have manually entered the number, with and without "1" before the area code. Nothing works. That friend is using Sprint, but I have other friends on Sprint that I CAN send texts to.
This is not a 8x issue, this is a coverage/phone issue for your friend. Either she is in an area where Spring coverage is not great or something is wrong with her phone. I say this because you can text everyone else on your phone and even other Sprint users as you have mentioned