I’m thinking in order to tie it into his character and maybe provide some closure to his ‘I will finish what Vader started’ bit, have him say ‘It’s finished’ or ‘It’s done’ right before she says ‘Ben’.
“It is finished.” Ben Solo out here about to turn into Jesus Christ lmao.
Anyways, that line “no one’s ever really gone” would be placed after he has faded into the Force and Rey is looking sad at his empty clothes. So there might need to be an ethereal filter applied. For me, it would explain why she suddenly looks happy while taking off in her X-wing, and also indicates that Kylo is chill with his uncle now since he’s quoting him. In other words, since we’re shown the other two OT characters forgiving Ben in this movie, I wanted to imply it with Luke as well since Luke didn’t really seem to give a crap about Kylo at the end of TLJ.
EDIT: Apparently the novelization actually has him saying “I will always be with you” and Rey responds with the Luke quote (which literally makes zero sense since she never heard it). I suppose just that alternative Ben quote could work instead.