You don't say whether he's an OTTB, but even if not, cantering past another horse like that is quite provocative. I would first suggest not doing that with any horse of whom you're unsure about the rider's control - such as this friend. I would suggest to your friend not to go out in situations like this until her horse is well trained enough to be safely controllable.
I would also suggest she teaches him the 'one rein stop'. I suggest it be started in an enclosed, safe area such as paddock, yard or arena, so your friend has no fear of being run away with. She needs to teach him to do it on either side, walk, trot, canter, then introduce distractions, such as yourself hooning past
and get him reliably listening to her in that setting, before *gradually* working from easy stuff to what you've described in other environments like on the trail.
Worse comes to worse, your friend could execute a "one rein stop" In which she takes her inside rein and sharply pulls it back to try to get that neck to bend. BUT.. a word of caution.. if a horse is in a full gallop and you snap at the rein like that
Sorry Skippy, but IMHO this is NOT a 'one rein stop' and it is a really dangerous - and unnecessarily hurtful to the horse - practice. It is not at all about 'sharply' 'snapping' a horse's head around. It is about TRAINING the horse to YIELD(respond to SOFT pressure) it's head around AND disengage his hindquarters(the important bit for control). It is about doing it so often, at standstill, walk, trot & then canter, that the horse doesn't even have to think about it, just does it. Only when it's well 'programmed' is it useful & safe in an emergency situation.
I only ride my horses in tom thumbs (or tom thumb style leverage bits/curb bits) and snaffles (most extreme being a snaffle mouth Kimberwick, for leverage in an english setting). If i need more than that, i've probably done something wrong in training! =)
Tom thumbs & the likes can be extremely harsh. Snaffles, if used with equal pressure on both reins can be quite severe, having the ability to 'nutcracker' the tongue and gouge the roof of the mouth. All bits can be painful if used strongly.
IMO control is always about training, never the choice of bit(from the perspective of how much pain it can create). You don't need to create more pain to control, you just need to learn how to teach the horse effectively & put in the time & effort it takes. I will not put any
bit in a horse's mouth, until after he has been trained to yield reliably without it, so that the bit can then be used for refined, gentle communication. I train in a halter or bosal & consider that if I ever needed more than that, i've probably done something wrong in training.