Answer
You must advise him to rise the oven temperature to 350 °F.
Work Step by Step
The temperature of 175 ° F is equivalent to 79.44 ° C, to properly cook a flan, the oven temperature should be 175 ° C or 350 ° Fahrenheit. The Celsius degrees to Fahrenheit degrees conversion is 1 ° C to 1.8 ° F.
(For a more detailed explanation look at Chapter 3 - Matter and Energy - 3.3 Temperature - Pages 69 to 73).