Either is correct, and the meaning is much the same. The use of "could" in this construction implies the immediate past, perhaps only a few minutes ago, while "am able" implies the present or immediate future, but "could" might well be used here for the present. "Could" suggests considering a different possibility where the person could not afford whatever it is. Both are probably using "She" to stand for whatever it is that "she" did -- provided money or resources perhaps, or relieved the speaker of a debt, or co-signed a loan, or arranged for a price reduction. In some way 'she' changes things so that what would not otherwise have been affordable became so, and that action is truly "the reason". "She" is used to stand for "her action".
The form with "could" is a bit more informal (because shorter by a syllable) and would be more commonly used in everyday dialog.