They want only the fries
If fries are a food they already eat, let the fries be the familiar part of the plate. Add a small piece of something else beside them.
Fries can be the yes
You do not have to finish the something else. The outing can still be a meal.
A few fries and a piece of soft fruit or a familiar bread, if wheat is fine, is a plate. The photo on the menu is not the requirement.
Beside, not instead
Put a small piece of another safe food next to the fries. Leave it if they ignore it.
Replacing the fries with a lesson makes the restaurant a fight. Keep the yes, add a little.
Not every outing
At home, dinner can return to the usual foods. Fries out does not become fries forever.
If the only side clashes with an allergy, skip the side. The familiar food is enough.
A sentence you can say
“The fries you like, and a little fruit.”