Good, valid points and suggestions from all those above...
I would add Lyme's disease
is now a problem near every state in this country today...and it is
effecting our horses too.
She fits much of the criteria and where you thought sleeping, she could have sensitivity to light hence "resting" and "sleeping" look to her.
She is also only 4 so still growing, maturing and needs special care and proper nutrition to meet her bodies needs of developing correctly.
Could be so many things...and a alert care-giver is often on to something before it becomes a anything.
I would put it to the owner as it doesn't sound like she is yours that she seems off and it concerns you.
Ask them to get the vet to do a physical exam, some blood work/chemistry to make sure she is really OK so the work you ask of her is not to much for her young body.
Some 4 year olds are "old in spirit" and just steady in their disposition and ways...
Knowing for sure she is OK,.. would give you & the owners peace of mind spending a few $$ for is so worth.