This is amateur hour nonsense. Sure, he'll notice his mistake and try to quickly unlock the phone to pull up the homescreen, but the damage had been done. I don't have to be Encyclopedia Brown to figure out that he's seeing someone else.
I didn't even have to say anything, I just shot him a look like, "Ugh. Are you serious with this shit? I totally saw that girl on your phone, bud."
And, he knew I saw because he got all squirelly and mumbled, "Uh, don't worry about that. So, what's your number?"
I was on the fence about giving him my number in the first place, but now with this silly sleazy screensaver slip-up, it's totally out of the question. Next!