Somebody asked my advice regarding this decision, and I'm am not quite sure what to recommend, I would appreciate your help.

The boat will be on a lake, no racing, just recreational sailing. No need to assemble or disassemble the boat very often, probably never. Kids are grown up and will be sailing with dad just occasionally, so single hading or two adults will be the usual (70 to 130 kg total, something like that). But 4 people on board occasionally would be interesting too.
No major interest in flying hulls or high adrenaline experiences. The lake is big and the winds are light to medium, it's more about lazy sailing, maybe go out far from time to time.

So it seems right to have reduced the options to the two mentioned. The dilemma is that the wave is the more practical choice, it will be much easier to take in and out the water, but: will it be big enough and as fun as the Getaway?
I know it's a vague question, but partial answers may be helpful..
Thanks

Sounds like a Getaway to me.

Geta...

For the days of higher winds, is it possible to sail it single handed by a 165 lb adult, seating on the tramp or on the wing (not on the trapeze)? Is it very stable or will it pitchpole or capsize just like other beachcats? (compared to Nacra 5.0 that I know, for instance)

the getaway is even more buoyant than the 5.0. If single handing he may want to take off the wings and use the trap.
If you get to sail on it the getaway will feel very flexible compared to your 5.0. Why not convince him to get another 5.0 into your area?

if single handing will be around 50%of the time then get the wave

everyone gets to have fun and learn to sail safely by themselves, righting it will be possible too

1 or 2 years after getting the wave if they feel they are outgrowing it or need more tramp space get another wave, or a nacra 5.0 or trade it in for a getaway

but start small and grow into it or risk being scared by a big heavy beast and wasting a lot of cash

That's exactly what he thinks, the only concern is that there is already a small monohull getting dust on the beach, similar to this one:

It's a small and slow boat, so it´s understandable, I'm sure the Wave would do much better, I just fear he might end up finding it too small pretty soon. But if the Geataway is not really well suited for single handing in a lazy way, with low chances of capsizing, it´s probably not his choie anyway.

The used you can find here are really old. A new one (N500) is more expensive than the Getaway and less ideal for what he's looking for I think.

About taking out the wings, do you think seating on the wing versus getting out on the trap would make a significant difference? I think solo sailing on the trapeze is not exactly the ideal scenario in this case.

Get two or three Bravos. We sail one with 370 lbs on it. Then we got a second one for taking more people out.

Can a grown man put up the mast solo on a Getaway. It is only 5 ft longer than a Wave mast but does that make it too heavy to put up? Can a grown man (185 lb) right a Getaway alone without a bucket or other special equipment?

Take a look through the Hobie 16 threads. There's been discussions on how to step masts solo. It can be done.

I'm sorry, but having sailed both (once) of the choices mentioned in the original post,

I'd say...neither one.

But if you must, the Getaway, getaway, getaway.....

The Wave reminds me of a Sunfish, only slower.

flame suited.....engaged