My heart goes out to you and the declining dog.. so sad and traumatic for you both. My worst experience was getting a favourite dog on The lead for a walk and the poor thing was so excited it died there and then! I thought afterwards what a great way to go! The pet owners were so grateful I dealt with it all.. ( I passed on the kiss of life opportunity! ) which the daughter suggested I should have tried!
They can go downhill very fast when they are old, sometimes they can be fine one day, and within days another problem has arisen, so it is very possible it had never happened with the owner.
Just in case you experience this again in the future (because every dog gets old), you can sometimes tell when it’s the end, because their spark/love/zest in their eyes vanishes. Their eyes say it all . It’s natural to get body parts failing as they age, same as we do, so there’s only vets that can tell from that side of things.
Given it’s older, the HO probably wants to take the poor thing to the vets themselves, to hear things first hand, rather than leave it in the hands of someone else to interpret what the vet says.
We love the oldies and the ones that need a little extra TLC, but you deserve a gold star, you’ve handled things as best you could, you’ve been brilliant
The important factor here is WHY the dog was collapsing : Was it orthopedic (significant but not life threatening) or cardiovascular/neurological (much more serious and possibly life threatening) ?
As a lay-person, you’re certainly not expected to assess the source of the collapsing but a couple of clues may be present.
The former (orthopedic) will present in a localised way - limbs buckle or give way, shakiness, possible presentation of discomfort/pain.
The latter (cv, neurolgical) is systemic - dog may be altered (confused, sleepy, acting abnormally before/during/after collapse), may lose control of body functions, hr and/or respiration may change, confusion, altered eating/play/activity patterns, loss of consciousness (“fainting”), and other signs.
Obviously you can’t tell as sit ended.
If you sit for a senior (dog, cat, horse or mouse) in the future be sure to ask the HO about the animal’s baseline (normal) behaviour - both physical and behavioural. This will you give you better insight if there are problems. Again, you’re not a vet but as a sitter, esp. when sitting seniors with health concerns, knowing what is normal for them is a great way to assess whether vet care is merited when things go wrong