Their relay list should be published in a kind 10002 which should be widely distributed, this means that you should be able to see what relays they use even if you are on paid relays which they are not paying for.

Unfortunately it's unlikely that paid relays are accepting these 10002 events (or kind 3) right now and its possible that you wont be able to find this users posts.

In this case Snort will query your own relay list as a fallback for that users posts, assuming your paid relays dont have anything then you wont see any content from that user.