Skipping over what others have pointed out - you don't interview a co-founder. a) ensure this is someone you are going to get along with b) get someone else to

Skipping over what others have pointed out - you don't interview a co-founder. a) ensure this is someone you are going to get along with b) get someone else to assess their technical competence - you can't assess it and the guidelines you provide won't work reliably.