A friend of mine tried spray foam on a remodel. As you can see the foam shrank, separated and pulled away from the studs. What did he do wrong?

Surface too cold?

mixture wrong?

Up stairs was sprayed with a different batch. Results were good.

I think in the back of our minds we all accept a minimal amount of shrinkage but this is ridiculous! I think some thing when wrong with the mix! Maybe temperature.

Up stairs was sprayed with a different batch

What does a "different batch" mean? Was it a different set of tanks? A different batch number from the mfr? Shot on a different day or one shot in the morning and one in the afternoon?

This could easily happen if you store product in a heated area, then bring it out into the cold and commence shooting. By afternoon, maybe the tanks have cooled some and it's enough of a difference....

I've also seen places (needing insulation, of course) where the top floor is adequately warm from the rising heat and the bottom floor is quite a bit colder.......

The different batch was a different set of tanks and a different batch number, older in fact. Southeast Texas doesn't get extremely cold but it may have been cool the day the lower floor was sprayed. I believe you are correct about the upper floor being warmer. I guess it's best to spray on warm days with the tanks acclimated to the ambient temperature.

Upper and lower were shot on different days.

This happens when you have bad chemicals or a bad mix in the gun. The temp really does effect the shinkage but will retard the blossum of the foam. Hot and humid give the best results for PUF.

Should you not use spray foam in a dry cold winter? I have a garage attic sharing same wall as a top floor bathroom that needs insulation. Do I need to wait til spring/summer for spray foam? Or are there precautions to take? ie. heat the attic for spraying etc..

heat the attic for spraying etc..

Yup. Read the directions and watch the videos for your particular foam kit. Temp is important. Those infrared electronic surface temp "guns" are pretty useful.

Put the units in an insulated box with a light bulb in it or some other heat source. Insulate the box and your foam line . The pipe foam should do for the lines. A temp close to 90 is best Spray some water on the garage floor to raise the humidity.  You could use a portable kerosine salamander to heat the space,  they throw off a lot of moisture