Changing what another player looks like, but only to you?

I’m trying to test something out in a game I’m making where one player can alter what another player looks like, but just for them. If you were in your own little “home” or world, and another player was a tall elf character and you wanted them to be a sci fi soldier to fit your theme more, you could change them into that. To them, they’re still an elf, but to you, they’re a sci fi soldier. How could you go about doing this? And if you did, is there a way to get a planar reflection or something to show them what they look like to you?