For this person to be colorblind the way they claim they are, there would need to be a history of colorblindness on both sides of the family (if the maternal grandpa was colorblind, for example, mom definitely carries the gene) and their dad would most certainly need to be colorblind himself (and if he's not, either oopsie poopsie that's probably not your daddy, or oopsie poopsie you should really consider getting a karyotype done because something's not quite right.)
This specific type of colorblindness is an X-linked trait, for anyone who didn't already know. There are other types that aren't, but red-green type definitely is.
Colorblindness is super rare in those with 2 X chromosmes (which, as an individual assigned female at birth, they would most likely have) because it is recessive, which means you need two copies of it in order for it to be expressed.
That's why it's always expressed in those with 1 X chromosome (because they only have 1 copy of a bad gene and nothing else to work with) and almost never expressed in those with 2 (because the good gene on 1 will pick up the slack from the broken gene on the other).
Basically, they would've needed to lose two coinflips in the genetics department in order to end up with deuteranomaly as a person who presumably has two X chromosomes, which is entirely possible, but extremely uncommon.
There are other ways genetically that I can think of which would've lead to them being colorblind in this way without a history of it on both sides, but that would be ridiculously unlikely and therefore I'm not even going to put it on the table.