I have a bike that had a stem similar to the Atlantis, and I converted it to a stem with a four bolt faceplate for easy removal. I think the new stem cost about $40, so the owner of a $2,500 bike probably can affford it.

If Grant has inspected the bike, and found nothing wrong with it, the "squirmy" ride is not the bike's fault. I spent a year or two riding mostly mountain bikes with very relaxed steering angles. When I came back to riding road bikes, I wondered what the problem was. I could not ride "no handed", or even "one handed" without the bike cutting left or right.

after six months or so of daily road bike riding, my body adjusted to the "acute" steering that comes with a 73 degree head tube angle, and I can now ride one handed again, or turn around and talk with someone behind me, while the bike stays solidly and precisely on track.

I suggest your sister put on the fattest tires that fit the bike, and ride as the lowest PSI that makes sense for her weight, and that tire. A lighter rider can ride on 28mm tires at 90 PSI just fine, and that will provide more stable steering than 23mm tires at 120 PSI.

Also, riding just one bike everyday for a few weeks allows you to tune-in to the unique handling of that bike. Riding a "different" handling bike just an hour or two a week is not enough to really make the marriage work.

But, if in the end, your sister does not want the bike, I will be happy to take it off her hands. I'll even pay for boxing and shipping.