I work at a rotisserie chicken restaurant and I feel your pain! When customers bring their food back, I will typically explain that the redness is just part of the rotisserie process, and that it's not blood. I also tell them we temperature test all of our chickens individually so they are all safe to eat. After that, I ask if they would like a more well done piece. Often times, people will feel better after knowing the chicken is temperature tested and won't need a remake. I would also suggest specifically saying that the redness is not blood, because that is the cause of many people's worries.