I'm flying with EasyJet and they didn't answer on Twitter yet: https://twitter.com/stefek99/status/891243760763494403
A quick search shows that they have answered to other users on Twitter, and the answer is yes, it's valid:
Question from a user:
Is it still true that if I get to my destination another way (not easyJet) I can still use the return leg of my booking?
Answer from @easyJet:
Yes that's correct, even if you don't use your outbound flight the return is still valid.