operating on C3
Not only will could your client become paralyzed from the neck down from a botched surgery at C3 but C3 actually helps to stabalise the axis and basically secures the upper cervical spine.
Just because a stretch may feel good, doesn't mean she should continue with it. There's a lot of things we do to ourselves because we "think" they feel good but in reality they are actually making things worse. You might want to suggest she mention to whomever is working with her on that about her stretching habit, she may even be making things worse. You're spot on about thinking that her cervical muscles are doing some healthy splinting. Those areas might not need or want to be "loosened" up. How much of her anatomy can she understand, does she have any understanding of her neural anatomy? Probably not if she didn't realize the importance of C3.
Depending on the details of her accident that created the herniation, you might not want to even work on the posterior neck, but check out the anterior and lateral neck and also the shoulders, mid-back and hips. However, you did mention the accident was a while ago? And she may not even have herniated discs anymore if she hasn't had any xrays since then (if it was truly many years ago!) Herniated discs can self correct over a couple years.