Allegation of Academic Offence
Well, you let him copy. What else can you say?
Bear in mind that lecturers hate hunting down plagiarism/collusion. In one course, hunting the offences and collecting proof cost me more time than marking all submissions (and these were a lot).
You just created a bunch of pointless extra work for them. Don't increase it, you have already been caught. This is not a court lawyer cat and mouse game. You are not going to get out of it free by technical tricks and it's insulting to your lecturer's intelligence to try, trust me on that.
Put the cards on the table and promise with contriteness never to do it again (and, of course, adhere to this). That's your best bet.