Is an Erased, Stolen iPhone, Safe on iCloud?
It is secure to keep it linked to her iCloud. It will remain Activation Locked, which makes it unusable. The data is not at risk because you've deleted it from the device, remotely. If this had not been done and Lost Mode was activated, you could have located the device.
Removing it from iCloud will, enable the thief to use it.
If there are no backups, iCloud or local, there is no way to retrieve the pictures.