Should Bitcoin Unlimited achieve 75% majority of mining power, it would become the “strongest chain” and define the rules for the network. By default, light-client wallets (SPV) and mobile wallets like Airbitz, are designed to follow the strongest chain. Therefore, they would not be able to recognize or send transactions on the “weaker” chain.
Source: https://airbitz.co/go/airbitz-and-the-bitcoin-fork/