I'd ask in r/Plex mate, it's a lot more active than here and they'll have better knowledge on Plex than most people

I think, the last time I was doing something similar, the traffic flowed through the server. So the upstream provider saw the connection coming from the servers IP.

But I don't recall if I was working with a plugin or something else.

RemindMe! 4 days "Plex m3u Ips"

Wouldn't it be more efficient (speed and cost wise) to put her on your Lan?

Anytime plex can be used locally you're much better off! And you could save money on her internet while you're at it, or, if money is of no importance to you, take the money you'd save by cancelling her account and upgrade yours to the fastest in your area.

I know personally my unlimited gigabit plan is cheaper than two 50mbps plans...

So you could probably save yourself some money and get significantly better performance...

Im gunna assume you have a reason for your current setup, but it sounds pretty inefficient to me.