He hasn't gone back on his word. He's brought up the idea to re-evaluate their agreement. He didn't demand anything. That's normal if an agreement isn't working out. And it sounds like if she doesn't accept different terms, he's still going to pay his agreed share.

However, she's showing now that she won't budge from an agreement even if, in principal, it was unfair from the beginning (which, if you believe expenses in a relationship should be paid proportionally, then it was).

So, the next time he makes an agreement, he should really insist on it being fair to him from the start. Since he agreed to pay 50/50 when it benefited her, he should accept nothing less than 50/50 when it benefits him as well.