Hi, I am not a teacher either, actually I am an ESL learner, but I think "suffers" has nothing to do with the issue at hand. The verb just agrees in number and person with its subject, "she", third person singular.
"...celiac disease and gluten intolerance, from which she suffers..."
Originally Posted by Nightmare85
***Neither a teacher nor a native speaker.***
The cluster has declined.
The cluster [of injuries from which she suffers] has declined.
Also, it says, "suffers", which would mean it's singular.
P.S. I'm not sure if the sentence is 100% correct, though.