In order for your client to retain her organization as a private foundation, she needs to continue filing Form 990PF every year, regardless of whether or not the organization has ever had activity. If the form is not filed, her qualification as a private foundation would be revoked. She would then have to reapply for private foundation status if at some point in the future she decided to actively participate in transactions.
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Since she actually had no transactions to report for those years then her status should still be retained, but the returns do need to be filed as soon as possible.