What is the difference between 做番 vs. 做返?
Both 做番 and 做返 are used to represent the Cantonese expression, jouh fāan.

There is a reason why some people choose to write 番 and others write 返.  Etymologically speaking, 返 is the correct character to use.  However, because the traditional literary pronunciation of 返 is fáan (2nd tone) instead of fāan (1st tone), some people will write 番 instead to make it explicitly clear that the 1st tone reading is intended (even though the original usage of 番 has nothing to do with 返).

Because written Cantonese is not standardized, both characters are typically seen; so your friend is not wrong when she said that both are okay.

BTW, the reason why 返 came to be pronounced in the 1st tone rather than its traditional 2nd tone is most likely due to changed tones, a phenomenon that affects various words in Cantonese (this phenomenon somewhat analogous to how erhua affects various words in Mandarin).