Yes, leave him to his grief -- only that bit wasn't supposed to have been said at all. Whoops. Jack didn't want some damn clone knowing just how much his relationship with Ianto had changed while they were stuck here in Vas.
Hell, he wasn't really ready to let the real Ianto know for that matter.
"And what --" Jack broke off. No, clones had the memories too. "And how do I know you didn't strip him of his memories?" Jack asked. "How do I know they didn't just take his brain and put it into your body? It's possible and I've seen it done, just with a bit more mess."
Coldly talk about Lisa. Now let's see how the clone reacted.