If they want to do a Camino, ie to the cathedral, then I also suggest Sarria to Santiago, and plan it so that they can manage each day's walk. Doing over a week will allow you to spread out the distance, but they would still need to be capable of doing around 16km/10mi on average each day. The other options that I have walked are the CI, CP and to Muxia and Finisterre.
The CI had some short, steep sections when I walked, but has been re-aligned since, and I don't have personal experience with the new route.
Walking back from Finisterre through Muxia would be an option, but is less likely to have the social ambience of the CF, and accommodation would be more difficult to coordinate.
Walking in from Tui runs a close second to the CF in my view. There was ample accommodation to allow for shorter days, and there were enough other pilgrims to give it a similar ambience to the Sarria to Santiago section of the CF.
Alternatively, if your thoughts are to do a camino somewhere, and not a pilgrimage to the cathedral, then there are many choices, including picking a couple of iconic days along any of the major routes and moving onto somewhere else. I have seen couples undertaking such a journey, I suspect with the help of a travel company. I don't personally see this as a pilgrimage, but it might be the sort of walking holiday that works for your friends.