I'm filming a speaker a week from today. I'll be filming with my Canon 70d a ways back from the stage. The speaker will be using a handheld microphone but I want to get the best possible audio recording.
If I hook her up to my Sennheiser EW122-p G3 Hotshoe Wireless Lavalier Mic do I need to worry about interference between the Sennheiser and whatever handheld mic she'll be using to project to the crowd?
No, there should be no risk of interference except possibly from operating two wireless systems (yours and the venue's) on incompatible channels. But you should be able to set your wireless system to avoid interference.
Consider also the possibility that you might be able to get a "feed" from the venue system.