I experienced this same thing with my golden...for many years. So much, that I knew when she was about to go into heat. The first time threw me for a loop, just like your friend. She was progressing great, and for two weeks would just not miss a beat. She was doing 200yd doubles and a blind across a pond and up the far bank, only to return, like "c'mon this ain't even a challenge". Then one day she balked at a simple mark....she charged out about 3 strides then gave up and turned to look at me. Lined her up again, "back"...she just looked up at me, like "meh".
Three days later she was in heat. Then, she basically followed me around, looking for answers to what/why she was feeling the way she did.
It passed and she was back to running just fine. Then, many months later, she began to be perfect on almost everything. Marking/Lining great, taking hand signals perfect....then she stopped again. A couple days later, she went into heat. This time she had a bit of false-pregnancy a month later, too. She huddled up a bunch of stuff in her kennel and tried to nurse it all. This enhanced focus followed by a shut-down period happened basically every time. Once it was perfect time during duck season and she was marking 3-4 birds and retrieving everything and diving for cripples. She lined a lively cripple damn near across a 500 acre flooded millet field that season and wore the duck out and pranced that sucker all the way back...longest retrieve she ever made. She was a little dot way out there, and on a mission.
I'm not saying this is what is going on with your friend's dog, but it wouldn't surprise me. Definitely have them talk it over with the trainer, and the two of them should have a game plan for what to do.
Best regards,
MT
