Note: I edited this to improve the quality of the question.
River Song put her life energy, expended "all of her regenerations" into the 12th Doctor in "Let's kill Hitler", and it was utilized - it saved his life.
Gallifreyan physiology and technology only allowed 12 regenerations before the episode "Time of the Doctor".
He meets old versions of his previous regenerations in "Day of the Doctor" implying that he gets to "revisit old favorites".
In the "Time of the Doctor" the pocket-universe Gallifrey gives the Doctor a regeneration energy that gives him a 13th regeneration AND regenerates his unused time in previous regenerations is put into the Doctor. He doesn't just get number 13. He gets 13-24 (or really 1-12 part 2). And here is where the question comes in.
The Doctor had expended his 12 regenerations in regenerating himself previous to Day of the doctor, but that is not all that he had done with them. Somehow those expenses were "recovered". Some of his own regeneration energy had been put in other places.
Now he had expended previous regeneration energy into River Song. He puts regeneration energy into River in "The angels take Manhattan". There are several places where his regeneration is used in alternate and potentially interesting ways. Rose, Donna, the Hand.
In him was more than his own regeneration energy. River Song put her life energy, expended "all of her regenerations" into the 12th Doctor in "Let's kill Hitler", and it was utilized - it saved his life.
He was dead, and in order to be alive required all of River Songs regenerations. The energy that restored the lost regenerations to the doctor, can it apply to her?
One might even be able to argue that they regenerated her regenerations and not his. Could River song have her lives given back or even enhanced due to what Gallifrey did in "Time of the Doctor"?
Speculation (hopefully appropriate in Sci-fi).
When he entered his own time-stream to rescue his impossible girl, the "dead" river song hologram/ghost mentioned "spoilers" - perhaps she isn't dead, and perhaps a regeneration would allow her to exit the library hologram system.