The property needs to be titled, the home made of non flammable materials.

Put nonflammable insulate around the vent pipe. 

The trick is to lay them out flat on a non flammable surface (like concrete) until they’re fully dry.

Backyard BBQ - make sure there's a spot away from the house siding and fence (if wood or plastic) with a level non-flammable ground surface.Fire pit - don't allow it and put this in writing in the rental agreement from the beginning.

Should I allow her to keep a small amount of these in the garage if she keeps them in a non flammable trunk?

Not ready to rent doesn't mean safe or nonflammable ;) surely a liability/ hazard thing

While many open and close cell formulations reduce flame spread and smoke over earlier versions, the foam itself is not a fire break and most are still combustable.When you use foam insulation in an external facing assembly, the primary role is to insulate and a thermal barrier is required between the living space and the foam insulation.In a fire-rated common wall between two separate dwelling units, the primary role is to retard the spread of fire (for 45 - 90+ minutes) as such, you want a non-flammable material - such as rock wool - in the voids of the wall.

Jack Cohen, a research physical fire scientist, advocates for home construction that better stops the spread of wildfires by including non-flammable construction materials and ensuring nothing exists between houses that an ember can engulf in flames.

Venting to the outside is minimal & we hid it behind the unit but you do need a non flammable base & that dictates the size of the footprint.