We travel to Europe at least once a year and usually twice. We have done ocean tours (Viking), river tours (Viking), and a couple of trips to Great Britain - one for a week in London and one where we used a British rail pass to tour England and Scotland. My wife uses a rollator with a seat. She cannot walk very far.
Here is my number one tip: Arrange for a wheelchair at each airport, then reconfirm at every step of the journey. The only time we ever had a problem with the wheelchair was in Barcelona. We managed to procure one even though the person at the wheelchair station threatened to call the cops if we took one! An empty threat.
My number two tip: the non-handicapped person should have TSA pre-check (a known traveller number). For travel outside the USA, you will also need Global Entry. These require an application and several months lead time; they cannot be set up the week before you leave! Be sure you advise the airline that you have one or both of these memberships. The handicapped person may be expedited through long lines, but sometimes the non-handicapped person can be left behind!
Be sure the rollator or wheelchair folds up into a compact package and is light enough for the non-handicapped person to carry. Be sure it will stay folded - that might require some sort of strap. Gate-check it at each stage of the air journey (a gate check means that you have the device all the way to the plane and it will be brought to the plane entrance when arriving - just in case the pre-arranged wheelchair does not materialize.
Have a wonderful trip!