Might I suggest (as said by a couple of other posters) that you say to your friend that it is becoming increasingly difficult to find an appropriate gift for her and as much as you appreciate her gifts, from next year you should agree not to exchange gifts, you could suggest the charity option or not, as you choose.
In the meantime, for this xmas (as it will be here in a matter of days and it's really too late to stop her buying you something), might I suggest a nice candle or if she keeps a diary/journal, maybe a nice notebook and/or pen.
Wine is a funny one if you don't know someone's preferences.
Plants and chocolate are easy options.
How about something from her favourite range of toiletries.
Are you a good baker? Maybe a homemade cake or cookies wrapped in a pretty bag might be good.
Hopefully, you've seen something that you think your friend will like from either my list or someone else's post. Good luck with finding the gift and I hope you'll be able to convince her not to buy anything next Xmas.