Or did she pay someone to take the test for her? (And I'm not denigrating anyone who has a GED. It's a valid credential that many people actively seek!)

You can't pass the exam without being able to read and understand what you're reading

But generally speaking, the identity of the student taking the test is verified during the process.

I myself worked with prison inmates as part of a GED and literacy education program, there was no way to impersonate another person in that circumstance.