I assume they are using the same internet connection?
Do you live in an area of the world where access to certain servers might be shut down abruptly (e.g., Anki syncing problem (“error sending request for url()”) - #12 by Danika_Dakika )? It might be beyond your or our control.
If you can’t get it working, you can rely on manual “collection transfer” for now. Collection Transfer - AnkiMobile Manual
For searchability: reading stream, ConnectionAborted, stream failure