Bule - that's a lovely way of conceptualising it.
I think that so often how we engage with people at its best is where either two people have a shared interest or where two people have complementary skills or interests and do things together.
Mind you, I realised a couple of years back that 'friend' is a funny word, because like loads and loads of words in English, it has a wide range of meaning. One person's 'friend' is another person's 'loose acquaintance' or whatever. We have a massive vocabulary in English, but not one, short word for 'someone I spend time with to our mutual benefit without either of us wanting to be committed to anything closer'.
I suppose where I am, the most useful word I have is 'neighbour' because if you call someone that, you can go somewhere together for a meal or shopping or sports or music or whatever without the other person or anyone else assuming you're trying to get too close.
As my mind drifts on this, I'm thinking of how lovely it is in general where you get people whose needs match in some way and each is the other's carer or supporter or partner or whatever. Which brings us full circle to your example.