lyra | sam
You don't have to talk to him about anything, but I don't think he'd mind answering questions like that. I've never actually asked, though I know angels can travel through alternate realities and through time. They've sent me back in time or into other realities before, and done it themselves. It seems like even people or beings who have ways of traveling through different realities have found that it doesn't work here, so I'm not sure if it would work for him or for your friend. I don't think it's horrible to wish he was here anyway, though.
I don't know. How long does it usually take to get sick from living in another world? If it's on the scale of years, I don't think anyone's been here long enough for us to really know.