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How experienced a cyclist is your relative?
Not very. And they're a bit of a technophobe and old hippy. But they're healthy. I guess in terms of weight I'm thinking about my experience lugging it in/out cars, stairs, through a stupid security door, etc. all without a locking rear. If it's just being lifted up/down a step or stowed in a side store box it's not that bad.
I think the 3 speed will be simpler. I've only ever ridden 6 speeds (S and M) and TBH I found them confusing at times and unintuitive. Although after serving my dad's RD, cables and going to town on the shifter with brake cleaner and silicone lube it was much better.
Although it's a big outlay even 2nd hand, the number of accessories is another selling point.
I think on balance I'm going to suggest a Brompton. Should also check what their camper is.
Thanks everyone, it's been really useful.
I was out on the Brompton (3 speed) today, because too hungover to be on the fixie. An old fella in full MAMIL on an expensive road bike was catching up with me at junctions for a bit and eventually, at a junction with a long light cycle, started chatting to me about Bromptons. He said he admired my turn of speed on the Brompton and he also owned one but he had a 6 speed. and felt he needed it. I said "Isn't the 6 speed thing a bit fiddly, with the two shifters?" and he said "Yeah, but I'm an old fella and I just need those 6 gears". So there's one data point.
(This is not a boast post, he was an old fella and I was racing between BTOB tags to avoid being tagzumped)
How experienced a cyclist is your relative? On the whole, I'd say the Brompton is just a solid piece of engineering that mostly makes life simpler than any other folder, but if they're not an experienced cyclist I'd definitely agree with @R.hobbs and say go for a lower-geared 3 speed, because the 6 speed is just going to defeat a lot of people and effectively become just a 3 speed for them. So they'll have paid more for a more complicated bike that they don't use properly. OTOH, if they're a seasoned cyclist, they won't find the two shifter thing a problem and the 6 gears really do seem to make a difference to some old knees.