Short answer
No, star fruit is not safe for a cat, regardless of its size. Carambola contains very high oxalates that can cause kidney injury in felines. Even a tiny kitten can be affected, so the risk does not decrease with weight. Call your veterinarian or the ASPCA Animal Poison Control Center at 888‑426‑4435 right away.
Star fruit — Food safety facts
Do not feed · Cats · full breakdown on the linked food page
Carambola is very high in oxalates and has been linked to kidney injury in pets. Do not feed it.
The full verdict, portion guidance and serving notes are collected on the Star fruit food facts page.