I'm nearing that age ... well a few years off. But I am in a position where, with full presence of mind, I have done things wildly generous to someone who has been unusually nice or kind or done something thoughtful for me which I had no right to expect.
You probably don't appreciate how important it can be to her be to be able to do something like that; to give a huge thanks in what she hopes is a meaningful way. And yes, your expression of appreciation is, IMHO, just perfect so far. You really helped her understand how much you appreciate her generosity, how much the gift means to you and that you are genuinely appreciative for her generosity.
Now you must be careful to not go any further and spoil it for her and possibly for yourself. Sometimes what goes around comes around. With grace, which you have displayed, let "Karma" work as it will. Continue to be attentive to her and her needs ... in other words, but the "nice you" that she sees and admires. You'll never regret it.
You need to understand ... this is not about money.
And, someday, find a way to "pay it forward". Only then will the gift be complete.