An interesting intersection between housing issues and health care, I know a couple who is currently looking at buying a duplex. One half for them is to live in, the other half is to rent to one of their parents. It addresses not only the issue of affordable housing, but also gives the couple an opportunity to care for the parents as they start to age. It seems like they're not the only ones with similar ideas. If you google "multi-generational housing" you'll get a lot of hits from various sources and perspectives.
Here in Los Angeles, granny flats are currently all the rage. The city has set aside a fund to help homeowners build second, accessory dwelling units (ADU's) in their backyard. As the name granny flats implies, the most common usage is to have people from the older generation live there, similar to your friend's duplex situation. In reality, it seems as though most homeowners are renting these out long term or short term on airbnb, which doesn't really help the original goal of the city's fund which is to help mitigate our housing crisis (shortage). There were some proposals that if you rent your back house out to section 8 (low income) tenants, you get all sorts of government rewards. I don't know if this program is working though, as it seems like a tough sell to convince homeowners to invite low income tenants into their backyards.