I do ride my boy when I'm alone, but only with the cell phone on my boot, and even then I know quite well that something could happen anyway and I make sure that I'm not pushing our limits when that's the case. But it's not always something you can control, and it's not always whether you're a good rider or the horse is a trained mount...Horse gets stung by a bee, spooks, trips, whatever - if you come off and whack your head, it can knock you out (helmet or no) and you won't yourself be in a position to call 911 if you need to. But that's something that doesn't even require someone who's horsey enough to snag the horse and lead it off - IMO, you just need someone who's capable of calling for emergency assistance. Being able to round up a skittish horse and lead it away is a +.
Or get another boarder...or - this is what I usually do - if I know that the BO is going to be off-property, I just make sure to show up when the stable-hands will be there doing the stalls. This is between 8:30 and 11am at my barn, and again between 3:30 and 5:30pm. All the barns I have been affiliated with (as an owner, a rider, a barn rat) had fairly predictable hours for when the cleaning/feeding/mucking/etc get taken care of. I don't need someone *watching* me ride, just someone who will notice that there's a loose horse and an unmoving pile of clothing on the ground in the ring, and will call for assistance.