I'm not an apple fan, but I imagine its the same process?

Someone please rectify me if I'm wrong......

Forward it, then delete it from his sent items, then delete it from the trash/recycling bin / deleted items folder.

That should work.

If his emails show up in a thread view (the whole exchange) double check after you've deleted it that there's no record of you being part of the convo.

Buuut. If you know his ipad is sync'd with his phone, don't do that, because his phone will tell him that he forwarded emails to you!

In this case I suggest you take screenshots, and keep the digital copies somewhere safe.