Sounds so exciting! A dream come true!
The number of horses that can be kept on 25 acres depends heavily on the land itself. We have 13 acres and two horses. I could probably handle a third, maybe even a fourth, but it would mean supplementing a LOT of hay. Because of those 13 acres, there has to be room for our house, garage, yard, pool, baby barn, etc. etc. We also have a garden, an apple orchard, a pear orchard, some watercourses and a lot of woods. In the end, we were only able to fence off about 5 acres for the horses. And right now, with my two small horses (14.2 and 14.3), they are eating it all.
Now, if you don't care about pasture space, you can obviously supplement with hay. However, you still have to provide enough room for the horses to move around. And be separated. It may not be possible to just have one big common pasture. I hate seeing boarding/lesson barns with insufficient space. It's sadly common around here (I'm in New Brunswick), and results in very muddy paddocks because the horses don't have enough room and grass never grows. So even without considering the need for pasture (these people feed their horses hay year round), it is undesirable to have too many horses in a small space. So I agree with your one horse per acre rule, as long as you don't expect them to live off it, but your 25 acres will shrink considerably once you put in the house, barn, indoor, outdoor, and consider what land is actually suitable for paddocks/pastures. And if you'd like your friend's horses to live part of the year on pasture, then I agree with what
says - 2-3 acres is more realistic.
A lot of people use no climb fencing if they want to keep horses, and other smaller critters like goats, in, and other animals out. Google No-climb fencing. It seems like it would be a better option than chain-link. But for my part, I am a huge fan of electric fences. It allows me to rotate pastures, and my horses completely respect it. I do it by the book though. 3 lines electrified (minimum), with ground. Mine are 5 feet tall. Probably more than necessary given that I only have short horses, but I'm not taking chances! My horses have never gotten out.
Not sure how much snow you get there. But here, we get tons, and electric lines get pulled down in the snowbank. So I have a winter paddock with top board (2 x 6") so they're safe and aren't tempted to step over.
You mention another large piece of land 20 minutes away. But to run a dude ranch/trail riding operation, you'd need to ride there. Riding on the road for 20 minutes with a bunch of newbies who think they're cowboys is a very bad idea. You'd have to figure out a path that doesn't involve riding on the road. That may be difficult, but look at the land to see if there's any possibility of getting right-of-way for a price.
Finally, you say your friend doesn't know anything about horses, and is just leaving them in the field and giving them hay. He needs to know that these horses need to be seen by a vet asap. Also, a farrier or trimmer (for shoes or barefoot trimming). Sorry if I'm stating the obvious and if this has already been done, just going on what you wrote. At the very minimum, a healthy horse needs to be vaccinated every year, and have his teeth floated so he doesn't develop hooks that will be painful, especially when wearing a bit in his mouth. Horses also need to be dewormed regularly. Even my two, with lots of room, a natural lifestyle, and very limited contact with other horses, get dewormed in the spring and fall. Ideally, you take fecal samples to a vet lab, they tell you whether the horses have worms, and what types, and you deworm accordingly. When my horses' fecal counts are zero (which actually just means they're below a certain number, but probably never actually zero), I deworm twice a year. Anything more, I deworm more frequently.
Of course your friend will have to look into liability insurance of some sort if he's planning on boarding or running a trail riding operation. Good luck! Let us know how things go!