Health, I respectfully disagree.
“If someone hasn’t yet been told to go on the GFD, there is no reason to start on your own before Pesach.”
The OP stated that her guest “has been diagnosed with asymptomatic celiac disease (discovered accidentally during an unrelated endoscopy).”
The guest is not a child who needs to wait to be told what to do. The guest has a diagnosis of celiac disease & needs to take care of it pronto via a gluten-free diet. Does someone with a dx of diabetes need to wait weeks till after Pesach to be told to monitor the sugar intake? It makes sense to prevent further damage physiologically.
We’ll have to agree to disagree!