In this situation, code reviews would be a great way to spread Objective-C/iOS knowledge throughout the team, so that someone else can fix bugs when the iOS developer goes on a honeymoon/quits in a huff/gets hit by a bus/is moved to a higher priority project.
Finding bugs is only half the value of code reviews. Spreading knowledge (about both the technology and the solution implemented) is the other half.
Besides, a decent Java developer may well spot logic errors/etc. in Objective-C code. Just the act of saying "So why do you do this here?" can easily cause the original developer to spot something himself.