I'm pretty thin so I can relate to this. I'll get cold fast and overheat fast too.

Unless it's freezing or below I use a lightweight wind breaker type jacket as an outer shell and a under it a fleece pullover or wool sweater, or whatever is appropriate for the temp. The fleece and wool layers are very warm but they're also breathable so they'll lose heat pretty fast on breezy or cold days.

The wind breaker slows that heat loss pretty good but once you're indoors you'll overheat in a hurry so pulling just that off asap will usually keep me from overheating.

Layering is a bit more work but it's also a lot more flexible so it's still the best option. She's really just going to have to refine her layers and get used to taking them on and off a lot. I know somedays I feel like I'm constantly doing that but it's really not a big deal when you think about it.