If you want to try it I think you should be able to do it Raspberry pi or equivalent on their network running Tailscale, use iptables to forward all inbound traffic on the Jellyfin port from that device to your Jellyfin server which is also running tailscale. Connect their Roku using the pi as the address for Jellyfin. I’ve not tried this with Jellyfin but I had something similar working with a Minecraft server