I have a mirror in my horses stable. I originally got one as there are only 2 horses on my yard and one has to kept on its own whilst I ride out. They do tend to stand by them when dozing. I also put them in the lorry for travelling them alone. I wouldn,t say they are a horse-friend substitue, but I think they help and they certainly don't upset my horses, as they choose to stand by the mirrors.
We had one in the stable of a welsh d who would try to climb out of the stable when the other pony went out, he would also push against the stable walls so hard you could see them move from outside (wooden stables). I think the mirror did help calm him down.
I put one up in my very stressy TB mare's stable about 5 years ago. She loathed it! After a night spent kicking 7-bells out of it and finding hoof dents and teeth marks gouged across it the next morning (it was a perspex mirror panel) I prompty removed it!
The above mare's filly, born 2 years ago, loves it though and is very happy with it in her stable. It definitely helps her to stay relaxed when she has to be in, and not weave as much when she is excited at feed time.
Before rushing out and buying one, I'd recommend considering whether your horse is a territorial type or not!
I got one for my stressy TB mare a few years ago when she was on box rest.
After we had put it up I read the instructions about not putting it next to where they eat - oopsy - mine was right next to the feed manger and the haynet... But my girly loved it and would stand and admire herself while she ate her haynet. I actually think it stopped her being so posessive about hay when there was some out in the field
My 3yo arab is in with it at the moment, and it has definitely helped him - he is very insecure and needs someone to hold his hoof all the time - he has definitely been more settled since he has had the mirror to admire himself in