I'm looking to buy tires off a friend (new and unused) with rims fitted and balanced. They're of the size 215/50R17 , which is exactly what my Honda Civic Touring has. He bought it for his Ford Focus, can I use it on my car? Is matching the size the only requirement?
If it's just the tires, then there's no reason why you cannot use them. This is as long as you are talking about the tires. If it's tires & rims, then the rims may not be alike. The bolt patterns & offsets are most likely not going to match.
I would love to add this as a comment, but I don't yet have enough reputation. I would just like to add to Paulster2's answer regarding wheel fitment. It is possible to check if wheels from different cars are compatible, when I was looking for new alloy wheels for my car, I used https://www.wheel-size.com. As you don't list the specific models or years of the cars you are reffering to, I can't look them up, however you should be able to find what you need here.
As others have said, the tyres will be compatible if you choose to have them swapped on to your wheel rims.