You shouldn't need to share relays.

Assuming their client published a nip65 relay list to one common/popular relay from where your client can get it, your client should be able to get their posts from one of their outbox (write) relays.

More details on https://mikedilger.com/gossip-model/

nostr:npub1gp4xzpmluelsakjtayc4wtzj97fhj5kakqjvsannu00xkdlf4x8s0xdqyq 's next release will have a working gossip/outbox model to address this exact issue. Will use new dart NDK lib written in cooperation with camelus client dev.