PEK wasn't flooded, so your thread title is a bit off. Certainly it was affected most of Saturday by rainstorms, which spilled over into Sunday getting things back to "normal."

And normal at PEK usually means delays for Air Traffic, even if weather is cooperative. There are two things going for them on this route, though:
1) The long haul flights usually come closer to getting out on schedule.
2) Heading eastbound, usually there is a positive push from the Jet Stream, so even if a delay of say an hour, the pilots can often make it up en route, put the pedal to the metal (so to speak) and arrive on schedule (sometimes a bit early).

There's a limit as to how much you can worry about things out of your control, all you can do is what you are doing...have a Plan B backup. One thing they need to be apprised of, if they don't already know, is procedure for doing the US Immigration/Preclearance in YYZ. I presume this includes picking up their bags there for Customs purposes, rather than at MSP? With only a 1 hr 20 min connection, this could be the bigger problem with making the connection, not any delay at PEK. I'm not sure if they have the option to skip this, focus on making the next flight, and just do incoming/US Immigration and Customs at MSP.