Please, get her something else besides a ski bib, nylon and dacron melts way too easily if she accidentally touches the ski bib legs to the hot exhaust and could produce a 2nd/3rd degree burn from the molten plastic.
I'm still cleaning the sneaker sole leavings on my mufflers from a passenger that put her shoes on the pipes instead of the pegs.
thermal bottoms and fleece lined cargo pants are easy to find and will protect from the heat instead of adding to the injury, or lined mechanic's coveralls.
Respectfully ... not true. She's been using the ski bid on ALL of my various bike for ~20 yrs. By the way, if all else fails ... use a drop or 2 of gasoline and it will clean-up your pipes from shoes sole marks. Works like a champ if something else like mineral spirits does not do the trick.