Timing belt changed, 2k miles later, belt broken because of "oil change frequency"

[AceAveo]

They've come back to us saying that the (new) timing belt broke because the oil wasn't changed enough.
This is weird, because the 1.5 years that my girlfriend's owned it, we've kept up with oil changes very well. Never going over 3500 miles before a change.

Well, then they're obviously feeding you a crock of B.S.. This is one reason why it's important to keep a record of all maintenance and receipts, it can easily be proven how often you've changed the oil. Their defense would crumble very quickly in the face of oil change receipts and odometer readings proving regular vehicle maintenance.