Answer
No, Henri not using the incorrect ratio .
Reason
As given
A restaurant makes vegetable soup using 22 cups of mixed vegetables and 25 cups of stock.
i.e

Now 5 cups of the mixed vegetables the number of stock required be .

solving the above
= 5.7 (approx) cups of stock is required .
Therefore Henri using the incorrect ratio for 5 cups of the mixed vegetables
5.7 (approx) cups of stock is required .