Yes, it's described in The Wild Silence, and it's on their first trip away together, where Sally is pretending to her parents that Tim's entire family are going with them, and she will have her own room. Whereas in fact they go alone, Sally stumbles up a mountain with a heavy, ill-fitting rucksack that rubs her shoulders bloody, to the horror of the campsite ('I couldn't give in, couldn't be that whingeing, whimpering girlfriend and ruin his longed-for trip' 😡). They then head off to the Fiddler, a mountain that Sally has had a bad dream about, despite her misgivings, are caught by a big storm camping overnight on its slopes, their tent and most of their equipment is lost, they spend the night huddled in a survival bag, on a site next to a lake that has become an island by morning, and narrowly make it back to the road next day.
SW presents it as a sort of Wuthering Heights Wild, Ecstatic Communion With the Elements (though one is slightly reminded of Adrian Mole's disastrous hike with Rick Lemon's youth club where they are told to make use of the bounty of nature, so he and Pandora take it in turns sheltering under an animal feed sack
), but it actually sounds like one of those dopey, ill-considered expeditions Mountain Rescue end up picking up the predictable damage from.