Survivors Benefits: How to Get Child Back Pay if Mother Didn't File Until 2 Months Before She Turned 18

So my girlfriend's dad died when she was 3 and her mother never filed for his benefits, 2 months before she turned 18 she made her mother file to help with college costs, is she eligible for back pay or how would she go about getting these benefits? Also her dad served as special forces for 20 years but didn't die while still in the military, so she falls through the cracks concerning those routes.

Survivors Benefits: How to Get Child Back Pay if Mother Didn't File Until 2 Months Before She Turned 18