It has been made clear to her that once converted to Islam it is an automatic death sentence to go back.
Psychologically speaking, she may belive that she is too tainted to go back to her faith, that because she gave in, no longer. can be forgiven.
She may not really think it matters what God she believes in since her Christian one didn’t stop Boko Haram from abducting them in the first place.
But you have to be really mentally broken to know what happened to your friends and embrace the god your captors did all that enslaving, coercion, and killing for.
She definitely sounds like she was very broken mentally