Ok, I'm one of those people. Have want for little with quite specific tastes and by my own admittance a nightmare to gift to as it's so hard to get 'right'. I appreciate gifts immensely for the gesture but genuinely I'd be happiest with a donation on my behalf so a family in a third world country gets a goat, or a girl's education is paid for a year, a local family get to have Christmas dinner, etc. I'm aware not everyone likes giving and receiving those but honestly that would make me the happiest.
Flowers are great anytime of year but at Christmas it's likely the house is already full of them. The same goes for food and drink items. Would still appreciate them regardless.
I think something small from Aromatherapy Associates might do the trick for your friend, they have lovely small Christmas specials that encapsulate a bit of luxury without bankrupting you. It's more mood driven rather than scent so you're less likely to misjudge. You should be able to source it online with a bit of a discount. Try feelunique or lookfantastic, the latter always has a 15% code on and free shipping. Be warned it is all rather tiny. A quick browse on ebay also brought up plenty of offerings.
If she's not snobby about brand names and you know her scent tastes and she appreciates a high quality bargain, the Zara Home scented candles are as good, if not better, than some of the more reputable brands imho. You can get a rather large one for just under £20 that fills up the room with a wonderful scent and is made with 100% vegetable wax, most of the high end brands can't even boast that.
A bit over budget, but if she likes her Jo Malone candles she'll love the Jo Malone Wick Trimmer if she doesn't already have it. Makes the candles burn better and last longer.
If she has a pet she adores, buy something for them. Guaranteed win.