yes and i read this on the canon site so its a no no for her lens.
"Extenders are only compatible with selected EF lenses, including the EF 70-200mm f/2.8L USM, EF 70-200mm f/2.8L IS USM, EF 70-200mm f/4L, EF 100-400mm f/4.5-5.6L USM, EF 400mm f/4 DO IS USM and fixed length L-series lenses of 135mm or higher."
ach well it was worth investigating.