Yeah he's never been a fan of hers but he knows she has powerful magic. He didn't know she could it. Remember he asked if it was possible.
and her initial answer was pretty much "no". but then he kept trying to convince her it could be done
last episode, when he said she might be able to help, I assumed he meant her helping them defend themselves against Thorne and crew. turns out he didn't need her for that (when I first heard all that noise in this episode, I though she was up to something)
and imagine if she did bring Jon back before Throne and crew were defeated. that's kinda how I thought it was gonna go down: Jon comes back, helps fight off Thorne, and then the Wildings show up to secure a victory
with Thorne already defeated, I would expect Davos to be rather practical about the fact Jon is dead, and they need to move on. Tormund was ready to set up a pyre, and burn him
Melisandre wouldn't come up with the idea because she just failed her king and cost him and his whole family their lives. Then she gets home to cry and sees Jon pop up dead even though she saw him fighting at Winterfell. The end of episode 1 showed that had completely given up on herself so there's no way she would be like "hey let try something I've never done before right after I've 2 of the biggest fukk-ups doing something I was good at".
I was thinking she might sacrifice herself in one last ditch effort to do something worthwhile. or
see something within him that would let her know it was possible, and/or needed to be done
but like I said, it will still be interesting going forward, as there could still be more to this then what we've seen so far