There’s nothing you have to do. It is your big day, after all.
As such, the question isn’t ‘do you have to return the favour’, the question is ‘do you want her in your bridal party’?
Whatever the answer, you should think through both sides of the argument and consider whether you’re willing to live with the consequences before making this decision.
Given your question, it’s probably correct to assume that you do not want her to be part of your bridal party, the question is why not?
Perhaps you’re not as close as she thinks you both are; you simply cannot afford to add her to your bridal party or you have too many others, such as family or close friends, who should be included first?
Whatever the reason, the truth is generally always the best route (even though it is sometimes the least comfortable one), so you should tell her and do so before her wedding, so she can make a decision and, possibly, replace you.
If, however, you know she’s going to be hurt – and that you’re not going to be able to live with that – it may be kinder to her and less stressful to yourself to simply let her be in your bridal party.
That, of course, is the easiest way out of this sticky situation, because the fact is, after the wedding, generally comes other things like christenings or birthday parties and you don’t want a repeat of this sticky situation year after year, so telling her the truth initially – and having the bridal party you actually want – is probably the best idea, but only if you can live with the consequences.
Keep in mind, too, that there are many other roles she may be able to fulfill during the course of your wedding, such as helping with a reading during the ceremony.