Seems like we had a recent thread on this general topic. And no I really don’t want to start a lively discussion. Mainly, in response to the OP, I agree with many of the responses, and further, yes, a martingale is pretty standard in some categories of horse use, so I don’t view it as a big deal. Particularly in view of the answer you were given- the friend’s horse is routinely ridden with the tie down, is used to it, that is ‘normal’ for that horse, so if it ain’t broke, why fix it, everybody’s happy.
I’ve relatively rarely had to ride a horse that needed a tie-down or martingale over the decades (though it is default gear in foxhunting circles and so sure, many loaners I’ve ridden come with one installed). And when I do, I’m sure not going to lecture the owner/trainer of said horse about holes in training (and besides, there are some great horses who just need one, end of story, too long to explain examples here). I’m just going to ride the horse with the equipment he’s used to.
Properly adjusted, the dangers of using same are vastly over-dramatized and over-stated by some.
As with any piece of tack or accessories like boots, the good horseman weighs the risks and benefits and uses what he/she needs, and no more.
If a horse requires a martingale to keep from breaking my nose or knocking me out, that for me outweighs the remote possibility of drowning in a stream (though I can always disconnect the thing when crossing a stream if there’s any question of depth).