Well it depends... is she a cold sleeper, does she mind a smaller bag or want more space. Where are you going and how cold does it get?
Assuming central NI, you can get frosts, but if she has a lightweight puffer that will add a few degrees so a bag rated around -3 to -5 should be ok. If cold sleeper go warmer.
If you want lighter weight, then you need a slim cut, better down (800+) and thinner fabric (7-10d) to get better warmth/weight ratio. Cheaper bags generally have roomier cut, lower quality down (550 etc) and heavier fabric (20d) . So they weigh more.
So a bag like the One Planet Nitrous -3 (572 grams) is in that zone, use that as a starting point and check out all the other brands with similar specs.
I would also consider a quilt - like those from kiwi ultralight I really rate my quilt, in warmer conditions just have it like a blanket.
Definitely shop around there will be something good on special.
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