Yeah, I've seen many instances of passengers asking another to exchange their aisle seat for a middle seat so the other party can sit together. In most cases that I've seen, they'd get upset at the thought of being asked or deny that request (probably frequent flyers). But every once in a while someone will try to help out at their expense of own comfort. Don't count on asking for aisle to window moves either!

To the OP, since you're requesting the change, it'd be up to the other passenger to move from his/her assigned seat. I did, however, one time notice a FA ask this middle passenger in E+ (or exit row, I can't remember) to see if she'd like to move to a row in the back to an aisle seat with an empty middle seat. At first FA's "recommendation", she declined. Upon a second FA's comment/"request", she decided to move to the back leaving an empty middle seat.