That's not true at all, though.
Anyone with a stick, a measuring tape, and the means to travel between two places at least 50 miles apart can recreate Eratosthenes's experiment and actually measure the curvature of the Earth for themselves. The validity of the experimental method can be verified by anyone who has a bit of high school geometry. No trust in NASA is needed to confirm that the Earth is not flat.
If someone wanted to see things for themselves and went about it in an honest way, they wouldn't end up as a flat earther.
... even before considering the transparently wacky claims of many flat earth hypotheses. Remember that to become a flat earther, a person has to adpt some sort of model of a flat Earth. I haven't found one yet that doesn't have holes you could drive a truck through.
Leaning over backwards to be fair, it could be that our
friend means that if all you have to go by is photos and
a globe, you have to have "faith" that they are right.
A bit of an aside...I know a lady from Philippines, who
is very Chanel and so forth now, in the US, but comes
from a poor rural background.
I was with her visiting her home and her Mom asked
(translated), "Why does it get cold in America"
and "Why is it day here and night there?"
I started in with an explanation, but my friend said,
"Mom does not know that the earth is round so she
is just going to get confused."