I haven’t tried this myself but I wonder if Jellyfin’s SyncPlay feature would work well for this. And as for adding YouTube videos, there are quite a few option

I haven’t tried this myself but I wonder if Jellyfin’s SyncPlay feature would work well for this. And as for adding YouTube videos, there are quite a few options, I would try Jdowloader. I’m not sure how it would work with your friend adding videos. Perhaps giving them access to a docker container Jdownloader instance with your Jellyfin directories mapped to it.