Run PiHole on a public facing server/port. You'll probably need to plan out the security aspects of it, but then your friends could then just set their devices to use the PiHole DNS (much like how people can set their devices to use 1.1.1.1 or 8.8.8.8).
Drop a Pi with PiHole in their network that is managed with some fleet/IOT management platform like Balena. That'll in theory get you PiHole running in a container, and you'll have a management platform to actually remotely connect to the Pi to manage it.
You may be already aware, but be mindful of the danger associated with having an ounce of responsibility for their DNS uptime. The bonus of option #1 is that you could also empower your friends to "bypass" PiHole should it go belly up.
Spitballing ideas:
You may be already aware, but be mindful of the danger associated with having an ounce of responsibility for their DNS uptime. The bonus of option #1 is that you could also empower your friends to "bypass" PiHole should it go belly up.