I'm afraid the hot water bottle just will not work. I know from experience, once the water gets cold it stays cold and so will she.
Don't bother with cotton shirts they don't work either. On the basis you kit up and warm up and sweat/perspire and then it gets cold,(cotton holds the moisture) and so will you and she.
worry not there are answers, and in the end only you can decide which is the best for you and she.
A one piece waterproof will stop the wind getting through and help keep body tempreture up.
Merino wool works both ways = cooler and/or warmer, dependent on the weather at the time.

Electrical gear also works. I use Kies stuff and am happy with that stuff, as I'm an all year motorcyclist. Kies do an independent battery so you don't have to rely on the bike for power, and she can stay warm off the bike also.

Good food in the stomach is well and far best to keep the energy/heat levels up.
Porridge for breakfast goes a long way. Stuff like energy drinks and chocolate, although they sound ideal, are not.
Spring is almost here, so you might start at the cheaper end, Liddle are doing some half decent underwear right now.

Stay warm. stay happy.

Socks

Oh yeh, don't know why but It appears to me womens feet get cold a lot easier than mens, and no matter the tempreture you might feel one can almost always find she thinks it s 3 degrees colder.