Another Brompton owner here. Think most things have been covered well – you simply can’t get something that’s feather-light, folds up into a tiny package and is actually acceptable to ride.
Which model did she find too heavy? There’s a 600g difference between the two models usually carried in bike shops (S2L and M3L), I went for the former as it felt quite a bit lighter to carry. You can save another 188g by going singlespeed, and 232g with Kojak tyres, although IMO I’d take the more puncture-resistant Marathons any day.
As said repeatedly already, you generally avoid carrying them as much as possible. Ride to station, wheel through the gate (rather than barrier) and along platform to your chosen spot then fold. At the other end, as soon as I step off the train I find a clear space and unfold then wheel along platform, carry unfolded up the steps, then through the gate before riding away. Can’t think of a time I’ve carried it more than 30m or so.