I have been fortunate never to have suffered a broken bone, but I have a recent history of taking significant falls in Europe that damaged one knee and gave me a couple of severely sprained ankles. The worst ankle injury affected me for over a month because I kept aggravating it. I now realize that I set foot on unpaved paths at great peril and know that cobblestones require great care. Some of my issues have occurred on less risky terrain when I was looking at the historic buildings around me rather than paying attention to what was down at foot level.
Rules 1 and 2: Do not hurry. Pay attention to where you are walking.
I think your mom can do this, but it will help if others here offer suggestions of nice cities and towns along their path that are flattish and don't have large cobblestoned historic districts.
She should strongly consider using a cane. If museums are on the itinerary, she could inquire about the availability of a wheel chair. If your parents enjoy museums, that will be something very workable for your mom since there are no indoor cobblestones. Historic churches should generally be OK as long as they skip things like crypts and bell towers.
There are lots of places in the London Underground and the Paris Metro where you need to go up or down steps, so they may need to stick to surface transportation. The other warning is that restaurants in older buildings often have the toilets either up or down a flight of stairs, and hotels in historic buildings may have no elevator (common for small budget properties) or may have an elevator but also some stairs needed to reach at least some of the rooms.
Will your parents be driving or taking trains/buses? Where is the wedding? How much time do they anticipate spending in Europe? That it's a summer trip will help a bit because it will probably be less rainy then than it would be in the spring.
Your parents should definitely be sure they have good medical coverage in Europe. Medicare will not apply, so they may need a short-term medical policy. And especially with your mom's history, medical evacuation coverage is essential. Heaven forbid she takes a bad fall and cannot travel back home in a normal airplane seat. The cost of a medical-evacuation flight (or even for a full row of last-minute one-way first-class seats) can be extremely high.
A lot of travel policies have exclusions for pre-existing conditions. Sometimes there is no exclusion if you buy the policy immediately after you make the first payment related to the trip. The website insuremytrip.com is often recommended as a place to investigate policy options.