If you have proof of return I've seen and heard of customs authorities overlook the 3 / 6 month requirements for passport validity. Saw it last year entering the US (not myself, someone I was travelling with) in fact. However, that is totally YMMV depending on the customs agent, the country, etc.
Assuming the AC agent doing the passport check in YUL doesn't stop you, you're going to enter the Schengen area in FRA. If the German customs agent decides not to let them in then (s)he's going to have a bad day tomorrow, being put on the next flight back to Canada.