A different person, route or pace can change a dog's toilet routine. Peeing on the walk does not guarantee they were comfortable enough to poop. First find out when they last passed stool and whether it is normal for them. If they strain, seem painful, vomit, have a swollen belly or go much longer than their usual pattern, contact your vet promptly. Do not assume this is a training problem.
Ask the walker to note where they went, how long the dog could sniff, whether the lead stayed loose and whether the dog tried to settle into a toilet spot. A familiar, quiet route may be easier than a busy long walk. Keep water available and avoid rushing them or repeatedly commanding them to go. The sniff-walk guide can help make space for sniffing, while the goal here is a comfortable toilet opportunity, not a longer outing.
Compare the routine with yours without blaming the walker. If the dog toilets comfortably with you afterward and otherwise seems well, the timing or location may simply need adjusting. If this is a new change or becomes a pattern of holding stool, ask your vet.
Today, share the dog's usual toilet spot and timing with the walker, then record what happens over the next outings.
Sources: Dogs Trust on recognizing toilet cues and not rushing, Dogs Trust on consistent routines, VCA on constipation warning signs.