The obvious solution is as follows: I will contract to warm your girlfriend on demand, night or day, anyplace within 400 miles of my home. Just call.
This being impractical, electrics are probably the answer. I assume you've already tried clothing her appropriately--that means lots of layers, functional water-and-windproofing, etc. Some pillions (and riders) are so attached to looking streamlined that they fail to take advantage of the undeniable advantages to the Michelin Man look. This is best viewed as a choice freely made: "I'd rather look sleek and thin than be warm." Electrics manage to sidestep the issue nicely.
Electrics do fail a lot. I've never bothered with them in part because whenever I ride with people who rely on electrics they've always got some sort of issue with them, usually involving connectors: "I'd be really warm right now except that the connectors on my heated clothing got fried last month and I'm waiting for replacements; do you mind if we stop at this pub for a couple of hours so I can warm up?"
Electrics also require a bike with sufficient stator output. If riding a bike which is marginal in this respect, think it through carefully.
Hope that's helpful.
Mark
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