Nothing has the same warmth for weight ratio as down. I'd possibly try a down gilet/vest to keep the core warm.
However I'm one of those folk who subscribe to the wearing as little as possible so I can warm the bag up more quickly view. I'll wear a base layer top, but if I am likely to be at the limit of my sleeping bag, I'll either throw my down jacket over the top or I'm often carrying my Alpkit Hunka bivi as an emergency shelter - this really increases the warmth of my bag.
The comments on mats are definitely valid.
Also, making sure you're fed and warm before getting into the bag makes a real difference. If you're already chilled, a few minutes of modest exercise makes a difference. Some people do sit ups in their bag but I just find that putting on my warm clothing and doing a few pre-bad tasks; putting stuff away, preparing stuff for the next day, is normally enough activity to get my body warmed up. If you're still chilled, a few things like arm-swinging can help.
And if she's still cold before going to sleep, a bit of snuggling up isn't going to do any harm!