Since it doesn't sound like your wife is under contract for a minimum number of hours, she has no real choice but to accept what the company is offering or quit. This is the basic nature of all non-contractual employment.
If she is afraid she is being lied to, the most protection she is going to get is to maybe ask for a memo to be issued or the employee handbook rewritten to state that 32 will be the new minimum number of hours required for employees to retain benefits. At lest then she'll have something to wave around if turns out a middle manager somewhere is trying to pull a fast one and the company really hasn't changed its benefit policy.