something will end up making her tired

Your question is about how to properly express the idea that 'something will end up making her tired' in Spanish. The given answer, "a ella le terminará/acabará cansando" is indeed a correct way to express this idea. This structure emphasizes the effect on her (making her tired), and it's a common and natural way to phrase it in Spanish.

Regarding your alternative, "a ella le terminará/acabará por cansar", this construction is grammatically correct, but it slightly changes the emphasis and meaning. The phrase "terminará/acabará por cansar" can be interpreted as 'will eventually tire', which subtly shifts the focus from the action's effect on her (making her tired) to the action itself (becoming tiring). While the difference is nuanced, the first option ("a ella le terminará/acabará cansando") might be more directly aligned with the original English phrase, emphasizing the impact on her rather than the nature of the action.

Saludos

Silvia