It depends on what shape your OH is (without sounding rude). I have curves (and muscles!) and therefore find the cut of mens jerseys all wrong. By the sounds of me, she is similar shape 12, with a chest to content with – in which case, I’d find womens kit for her!
In my jersey store at the moment I have:
Mavic Meadow – very comfortable, well fitted but maybe a little thin for what you mention. I have a couple I use all year round and great in rucksacks, under a long sleeve thicker jersey or warmer days.
I have several tops that dont have zips: Endura, Fox, Loeka. The Loeka is very freeride fit – great on warm gnar days but a bit baggy. The Endura one (I dont know which one it is as was given it as a present) is very comfortable.
I have become a massive Gore Bike Wear fan girl of late though. I am slowly (!) replacing a lot of worn out stuff with it – I have the Alp X short sleeve top and long sleeve thermo. I’d see if they do one in the sort of weight you are interested in – doubt you’ll go wrong. Evans had loads on offer. Note, on thier website, some stuff comes in slim fit, some in more relaxed! Buying womens kit is a bit of a minefield…..! I have bought a lot of large (40) stuff of thiers in slim fit (the Alp X and Power ranges) – they then are slim round my stomach, but allow for the chest!
I have used this as a good excuse to go back to the Gore website and make you a combo table Gore goodness!