Upvote:4
Yes. It's breaking the terms of the agreement between you and the country of the United States of America.
Getting out won't be a problem - there's no exit immigration, so she'll be able to leave. However, overstaying is usually frowned upon, especially by the US, and it could cause her to have problems getting future visas for the US, or for other countries if they share the information.
Best option is to change the ticket, or apply for a longer visa than the 90 day VWP option.