So you’d rather cut short a walk than share your lunch?
that's not at all what I said, is it?
To recap, I said, if I didn't have lunch with me and my friend only had a small lunch for one with her, I'd go hungry and be fine with it for one meal. I'd wait till I got home and have lunch there. Half a sandwich and half an apple, really isn't going to make enough difference to me that I'd cut my walk short, if I was looking forward to it.
You asked then, what if your friend was the one who forgot hers?
So, trying again to answer your question here; if my friend and I had met up for a country walk and said we will each bring our own lunch and we had each made a single portion lunch, like maybe a sandwich and an apple each, with a bottle of water. We arrive at our destination and lo and behold, friend has accidentally left her lunch behind.
Are we assuming that the walk is long enough that we couldn't just forget about it and have lunch together afterwards, as I said I would if I didn't have mine with me? If not, that would be one option.
If that is not possible, then I'd say should go for a walk via a pub / cafe and buy lunch there.
In that case, I'd just leave my lunch in my cool box or wherever and eat it for tea later.
If budget was an issue, I'd say, we'll buy lunch for one at a cafe or pub and share the two lunches between us. That's another option.
If friend said no to both of those options, I'd say fuck it, let's go home, have lunch at mine / yours and we'll do the walk later or another day.
If we were so far from home that we couldn't do that, then we were a bit silly for coming out somewhere where we couldn't procure any food with nothing with us other than a sandwich, an apple and a bottle of water each in the first place.
If we had EXTRA lunch, then we'd share that.
But this^^ scenario is indeed, to use your phrase bertrand, a strawman argument and is nothing at all like the op, so I'm not entirely sure where this whole discussion is going...