First, it's not clear a contract was established. And even if it was, unilateral changes generally are unenforceable. And even if it were there when the attempted purchase was attempted, this is an unconscionable contract clause, against public policy (1st amendment, etc) and should be thrown out.
This person's best bet is to dispute the credit reports, counter sue for whatever they can think of to recover legal fees.
If it were me, I'd just send them a letter telling them to go F themselves and I'll see you in court. Bring it. My lawyer, however, would likely wish that I not do that.