Great Stuff Foam used intead of end cap ? Dangerous ? Okay ?

[Lele H.]

Hired contractor to remodel kitchen.

Needed vent pipe capped off .

He was supposed to cap off an open hvac pipe that is blowing hot air from the ceiling.

It used to lead to a heating vent in the old wall that was removed.

Instead of using foil duct tape, he sprayed "Great Stuff" foam INSIDE of the 8 inch in diameter active vent/pipe.

I told DH to turn off HVAC heating to avoid drywall dust. Sub didn't even mention turning anything off.

I have looked and looked online but can't find anyone using foam to seal off the entire end of an HVAC vent/pipe.

His excuse for not using the foil tape he had was that it would stick.

Anyone ever use Great Stuff to seal off an entire vent ?

It says it uses a highly flammable gas to expel it form the can and that Great Stuff itself is hugely flammable.
Should we be scared to turn the HVAC back on ?
He used ton in the hole ?
Is this really bad form to use that intead of an end cap ?