Some welcome news for a change. I hope that there is some sentence attached to this plea, ie that she isn't free to plead guilty with no further consequences, but I'm not sure what the next steps will be.
She could most likely expect a 3 or 4 year sentence were she in the UK. She has most likely been given some sort of assurance by the Americans that she won't do time if she pleads guilty.
From them, as she is American and used to driving on the right hand side of the road, I would suggest this is appropriate:
"Careless or inconsiderate driving arising from momentary inattention with no aggravating factors"
So a community service order.
we saw a car pull out of a hotel in front of us in the lake district drive on the right towards an 35 degree right hand bend i flashed headlights constantly sound horn to no avail, obviously a tourist from a right hand drive country very luckily nothing came round the corner as it entered it, very unnerving for us both expecting the worst