Do you want your indoor arenas and outdoor arenas close to each other? Would you put the indoor and outdoor so that they couldn't be seen from each other? Or, would you want to have the outdoor visible from the indoor?
For convenience my biggest preference is that they are located close to the barn, especially the indoor if you're in a climate with less that ideal winter weather. As far as near each other vs. out of sight of each other... You'll have to decide but speaking for myself, I prefer them within sight of each other for training purposes. At shows there are usually multiple rings going, I think it's helpful particularly for the greenbeans to get used to keeping their attention on the rider even when there are other distracting things happening.
When I built my barn/indoor, I made certain that I could just open an indoor arena door and walk out to the outdoor. I didn't want to have to tramp through mud (clay soil here UGH) and then have the clay dry into 'nice' ankle-braking rock hard hoofprints getting from one to the other. It came in handy when you had a rainy day cause you could easily just walk into the indoor if it started to pour.
Also, I used to board at a farm that you had to traverse a muddy paddock to get to the outdoor so half the time you didn't to ride outdoors until June.
I'm not saying let's go kill all the stupid people...I'm just saying let's remove all the warning labels and let the problem sort itself out.
I built mine so that it's close. I do have one arena that is farther away, but that's because it was there. I have a friend who is doing it in an opposite way. The outdoor can't be seen and is some distance away. I just thought it was strange and wondered if i was missing something. My plan is to allow people to watch the arena, but that may not be important to others. Thanks for responses.
We built our indoor just down a slight hill from our barn. That is just the way the land was...its about a one minute walk. My outdoor sand ring is past the indoor arena, past a few paddocks and located out in the back field. But I love it there! Its spacious, away from the other horses and makes for a perfect riding and training enviornment. The plus side is that the walk down to the ring is a good warmup and the walk back is a nice relaxing cool out.....the driveway down to the sand ring is crushed asphalt and always excellent footing, even after a heavy rainfall. I cannot imagine having to school close to the commotion of a barn.....
Definitely close, I think your friend will regret it later. My FIL built one further away and I don't think he uses it nearly as much as he would had he put it closer, it is just a PITA. You have to go down a 1500 ft + driveway and around the corner. Just not convenient at all.
I prefer the riding areas to be visible; we had a horse fall with a rider in a blind area, if the horse had not been taught ground tying the out come could have been much worse.
Once it was noticed the rider had not rounded the turn from behind the barn a person went to investigate to find the horse standing with the rider's foot twisted in the off side stirrup .... nearly every bone was broken in the foot.
Afterwards we fenced off that section into a corral to use as a holding pen.
I managed a farm where we put in both an indoor and outdoor (it had neither when it was purchased). They were side by side, mostly because it was the one spot on the property that wasn't going to require 100k worth of excavating to level the spot (as it was, we still had to dig into a hill and build a retaining wall for the outdoor). They were both a walk from the barns, but not too bad. Could be interesting after dark, but not too bad.
Now, I am at a place that has the indoor right next to the barn, which is nice for the really crappy days. The outdoor is a short hack away.
The biggest issue we had with rings next to each other was sometimes the horses in one ring or the other would spook at horses or things in the other ring. The indoor was all windows, so it was like riding in a glassed in covered ring.