Guys, re-read the part where she mentions that she can't find a suitable frame at the moment and wants to use a frame that she already has. Not to mention that she has unique fit issues so the regular fit frames don't fit well on her in any case. It's also for use as a sloppy weather bike, not something to win races with. So there's nothing inherently wrong with a bit of franken'biking in a case such as this.
Moving the seat forward would mess up the overall body balance and try to shift too much weight forward onto her arms. Those old tyme mountain frames were set up with more agresive angles in the first place. It's not like today's mountain bike geometries that often have a pretty laid back seat tube angle. Granted SOME fudging on this aspect to tune the riding position to the rider is always a good idea but I don't think she wants to be adjusting it more forward than she needs to try to make up for a long top tube.
Using one of those short downhill stems or other rugged short mountain stems set backwards would be fine. There's nothing dangerous material wise with such an arrangement since those parts are so grossly overbuilt to resist all the crashing that is common. And used on a sloppy weather commuter they'll never see even a fraction of the torture they are designed to withstand.
From a steering feel dynamic standpoint it'll feel a bit funky at first but again I don't see it being dangerous. Your grips will end up at a point that is no worse than what you get with many easy upright riding hybrid setups and certainly the final hand position would not be back as far as we see with those pull back beach cruiser bikes. So while there would be a bit of getting acquainted with the feel of the new hand arc that's all you'll get. There's certainly nothing at all inherently dangerous about this since you're just altering where your hands sit in space compared to the front end and not the rake or trail of the front end geometry. Various bar and stem shapes are just a way of achieving this goal. None of the components are going to suddenly crack because they are used backwards in this application.
Just a thought. If you do get one of the short stems keep in mind that it'll have a negative rise if mounted backwards. For this reason you may want to look at the short ones with a 45 degree rise so you still retain about a 35'ish degree rise when it's facing backwards.