Just read over your 2nd msg OP - where you set out their convo.
He knew that he'd lost his mobile at her place.
He asked her and she said "just checked, it was under the sofa".
If their story were true, which it fucking isn't, then even if the beginning of their convo went like that, the rest would go "under your sofa, wtf is it doing there?!!".
He didn't lose it at the venue. She didn't pick it up at the venue for him.
He went back to her house and spent some time, one way or another on the sofa.
Now, I'd be a lot more inclined to believe that he went there for a catch up/night cap with friend/s - but the lies say otherwise.
I suspect he's going to say something slong the line of "well, I knew yuo'd go mad if you knew I went back to hers for a drink and ended up passing out on the couch"...because you're not dumb and he'll know that.
Tell him you know the truth (don't say how, I mean, I know it's just a gut feeling at this stage, but you could let him think that perhaps you had other evidence) and he has until 8pm to tell you the truth - whatever it may be..