I would stop them. It would only be to inspect the quality of their rope, examine both the noose knot and the anchoring knot, check the sturdiness of the anchor point, and ask them if they know about carotid constriction. If all is well, I'd then walk away and allow them to carry on. I don't really care either way whether or not somebody, regardless of age, intends to CTB; it won't have any effect on me and therefore my opinion on the why of their attempt is irrelevant. It's also best to respect the bodily automomy of others and give them the benefit of the doubt that, whatever the reason, their reason for hanging themselves is a good one.
Given that hanging is also my chosen method, I am more concerned with the how and do believe that my opinions on matters of method are worth listening to. I've had the misfortune of watching quite a few videos of hangings in which the subject thrashes about for several minutes before losing consciousness (windpipe constriction without carotid constriction) or completely failed attempts during which the rope either snaps, the anchor point collapses, or others intervene because the subject made too much noise.
I just cannot stand the sight of a sloppy CTB. That is all.
Thank You,
CumbriaCTB