Italy in 15 days into and out of Milan.

Mike, how complex is the itinerary that you would have followed together if you'd been able to go? What would be the biggest challenges to doing it on their own?

Rick Steves has excellent packages but the dates do not line up, and
the destinations are not what is desired.

I think they're going to have this issue with just about any organized tour. What are their desired destinations?

Editing to add, I'm so sorry about your wife's back problems! I'm sure you both were looking forward to this trip but these things can happen so you certainly didn't "create a mess". :O(

Is driving in Italy a big issue for your friends? Did the itinerary you developed for them involve places that could only be reached by car, and if so, could it be modified to involve train only?

Have they been anywhere on their own before? Are they competent, capable people? Milan has train connections to so many cities. Perhaps they can do some of it alone and hire local guides for some of the spots.

Have them come for a meal and some time, and share some research tools with them. RS videos, Seat61.com for train details. Youtube will have videos of 'how to' (take a train in Italy, order a meal). English is so extensively available that it shouldn't even be in the equation.
I can understand feeling somewhat responsible for backing out and leaving your friends with the perception of being stranded, so if that is the case, then perhaps help train them up. You know, 'teach a man to fish....' thing.

With some great discussion and help, they could possibly (depending on where y'all were headed) do it on their own if you help prepare them. Are they specifically worried about going it alone?

I believe Perillo Tours will design custom land packages for travel in Italy. It could be just transit and hotels, or it could include some local guides too. I doubt it would be cheap, but at least there would be no communication barriers setting it up.

See if any of the dates for this Smithsonian tour (or one of their other Italy tours) lines up.

https://www.smithsonianjourneys.org/tours/italy/pricing/

You will have to talk to the tour provider about dropping the flights from the package and meeting the tour in Italy.

Also try Road Scholar, and Odysseys Unlimited. Or put together two shorter tours with HF Holidays, such as this week in Tuscany ( visiting Siena and hill towns) plus a Venice "city break".

https://www.hfholidays.co.uk/holidays-and-tours/guided-discovery-tour-tuscany/

We have used HF Holidays for walking Holidays in England and really like their approach.

Since you have already done all the planning, can’t they do this trip independently. Traveling in Italy, especially in the north, is not difficult at all.

If giving them your detailed itinerary and letting them do it themselves is not an option, then perhaps they just have to pay the change fee on their air tickets and eat that comparatively small loss, rather than having a miserable trip.

I would like to thanks everyone that offered suggestions to our dilemma. Our friends have been able to prevail upon a friend who is also a travel agent to fill MOST of their needs. They will just need to get themselves from Milan to Rome, which they think they can handle. Thanks again for all of your thoughtful ideas.