When a baby or toddler has pooped in their diaper they really should be changed as soon as you notice or as soon as you can change the diaper.
There have been times when I was out and about though and had to leave my 3 year old in his diaper that he pooped in because we were nowhere near anyplace where I could change his diaper.
Add to the fact that I had forgotten to bring anymore diapers anyway and I had to get the errands done in town and I was not near the store to buy his diapers.
I had to first go to a doctors appointment for his older brother and so the 3 year old had to sit in his diaper with poop in it until the appointment was over with.
He didn't seem to mind the poop in his diaper and other than the poop the diaper was still dry and could hold some urine.
So after the doctors appointment I went to the store with him and got him a pack of diapers and then went back out to our van and changed his diaper.
After that day I made sure to keep extra diapers for the kids in the van and in my car so that I could change their diapers when needed.
Poop can irritate the child's skin and cause a rash so yes you should change the diaper after every poop but if you cannot get too the poop diaper right away then you're not alone.
The child will be okay.