My take on adapting old lenses may be a bit different, from my own experience. There are 2 questions you might need to ask to get a better idea of what your friends want: what's the budget and what are the expectations of the result (Small print, Large print, Online sharing, Pure curiosity, or for pure fun).
It matters because the higher the expectations to more money, time, and effort need to be put in to meet those expectations.
The most basic I can recommend, and for fun, is:
*Pentax 110 lenses and C-Mount lenses... Cheap enough, you can get a decent variety of FL and the IQ is good enough for smallish prints and online sharing. For cameras I recommend any Micro Four Thirds that has Focus Peaking (it can help a lot with manual focusing, especially for beginners) and if possible and budget allowed IBIS, it will increase the IQ results in all conditions, not just daylight AND it will reduce the frustration for beginners who might not understand why the results may be blurry in low light.
*Any dumb adapter (no electronics), solid metal preferably as some lense can get very heavy with most legacy lenses and a FF mirrorless camera. It can be any Nikon Z or Panasonic S or Canon R or Sony FE BUT Sony A7 is the oldest and the cheapest you can get (ca go as low as 400£ used in some places). A Sony A7 II might be a better alternative for the IBIS if the budget allows it. I would not recommend any camera higher the 24MP because its a strain on pretty old optics that won't stand well to such pixel density (film grain is different, it can give better results from the random nature of the crystals in the film compared to perfectly identical pixels size and shape of modern sensors).
If you want cheap old lenses look for the forgotten mounts or the oldest mounts, like M42 or Yashica/Contax. Some of the most expensive mounts are the highest quality optics or rarest lenses or mounts that are still compatible to this day, like Pentax K, Nikon F, Minolta A.
Personally, I recommend primes over zooms for anything pre 1990s on any mounts because optically most zooms have less image quality and even loser quality control, except for some extremely expensive lenses like the Yashica ML zooms and Carl Zeiss zooms.
Another recommendation is to not look for IQ as much as look for personality, character, optical qualities: color rendition, swirly or soft or smooth bokeh, and other qualities. Sharpness is not everything the same way blurry background does not make a picture amazing *cough* Tony *cough*.
Also, think about fun, not all old lenses have the same amount of fun, some can be more frustrating to use either by old design or mechanical faults like grindy or stiff manual focus, stuck aperture blades, loose controls like skipping aperture rings, wobbly manual focus rings, etc.
One of the best places I found for reviews on old lenses is this one
https://phillipreeve.net/blog/
(Personally, I don't think it's much worth using APSC cameras with legacy glass even with speed booster because it has higher density pixel to beat the IQ out of these old optics. It would be like trying to watch the TV with a magnifying glass

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(With Micro Four Thirds it's a bit different if you stick with Pentax 110, Olympus PenF, C Mount, and 16mm cinema lenses because they are designed for smaller film size).