Are they riding in Western saddles or English? They would be safer to get off and lead their horses down. The more slippery loose rock and trippy stuff there is on a steep slope, the safer it would be for the horses to only have to balance themselves and not a rider.
If safety wasn't their main concern, like they were on the run or something, they might just ride down. But that would not be wise.
How many riders and horses are there? If there is a great deal of loose shale, going single file means someone uphill might stumble and send rocks rolling down to trip the ones ahead of him, so they need to spread out (nobody walking behind anybody else), unless there is a certain route down the side of the wash that looks safest. In that case, if they go single file, they need to leave at least a couple horse lengths' worth of distance between each of them.
The riders need to keep the pace slow, and lead their horses with loose reins so that the horses can have their heads and necks free to balance.
They need to walk beside the horses, not right in front of them. Although that actually probably goes without saying.
If anyone does stumble and fall, let the reins go and scuttle out of range.
The steeper the slope is, the more you would have to zigzag or go down at an angle, and not go straight down. The Man From Snowy River notwithstanding.
You may have already hashed out some of these details. If so, feel free to ignore any repetition. Basically, dismount and lead, go slow, and don't let anyone crowd each other.