I fly alot , and there are a few factors to be concidered.
What kind of plane is she flying? If it is a small regional jet the overhead compartments are REALLY small and would not accomidate the machine, leaving the underseat area. The underseat area on the regional jets are small as well. So if she has a carry on she is planning on putting under the seat , the machine and the carry on will not fit.
If she is flying on a larger jet , the biggest consideration is when she will be boarding, if she is in a boarding group that is one of the later ones to board all of the overhead space may be taken , especially during the holidays. This would leave her with the under seat area in front of her seat.
On the regional worst case is if she can't fit it she would have to gate check the machine. She carries the machine all the way to the gate , and then it gets loaded via a guy who takes all the bags that don't fit and puts them directly into the baggage part of the plane. She would pick it up on the jet way when she gets off the plane. Gate checking does not cost anything , and it bypasses the typical path checked luggage takes. It is a BIT safer. It does count as a bag though , and the arilines allow two bags as carry on ( this count includes a purse). A typical feamle passenger would have a laptop and purse, this would count as two.
I am not sure I would ever trust the people in baggage handling to properly handle the machine if you checked the machine the traditional way.
Hope this gives you some info.