Yeah, a proper solar setup would absolutely work - I did something similar with a Pi zero 2w and Motion OS. Actually Pi’s are fairly nice webcams for some speciality applications - e.g. when you want to use multiple cameras from the same device, need special cameras or want something done appliance-sided. And the form factor is decent with the RPI02w.

I intentionally didn’t use a solar panel as the main power source, but used a regular large USB battery with an analog timer/clock that simply powered the pi on for 20min a day and gave it enough time to take the picture and mail it to me. (I had WiFi at the site)

The solar panel was only an add-on to increase the time the battery lasted but it worked pretty well during the summer months. During winter it didn’t do too much, but that’s okay - I still got two weeks out of the USB battery which was pretty comfortable. I simply replaced the battery on the days I was on-site and never had a problem.

I now power a Lora to NB-IOT gateway with a complete solar kit, though with one of these camping kits that provide 60w and charge a battery. The NB IOT gateway takes 12w max(4w on average)so it is more than enough to both charge the battery and run the gateway even on cloudier winter days. But this project is in a pretty sunny spot and it’s placed outside - I would highly discourage you to use one of these in your friends house - the first set I had almost caught fire. No biggie in that location, huge problem if it is your friends home.

PS: Do you have a chance to use PoE maybe?