Just read the previous answer. I sure would have loved a spa day when I turned fifty! But I'm not saying how long ago that was.
If you want to include the number in the gift, instead of some sort of gift card you could give her fifty silver dollars, or fifty gold dollars. Or fifty roses. Or fifty iris, if roses are too pricey (which they will be if your friend's birthday is before Valentine's Day). Or fifty chocolate truffles, if her tastes go in that direction. Fifty is a GREAT number to be!
Are you able to contact her relatives and longtime friends and get pictures of her? You could do an album about her first half-century and how you're looking forward to her second half-century.