I can't answer that Robbie.
If the immigration journey for Guernsey follows that of UK then your wife be required to reach 30 months living in UK to be eligible for application of the second period of 30 months FLR
Better check with your immigration guys over there.
As it stands (albeit against UK rules) it appears that your wife cannot reach the required 30 months. (it's allowable to make application max 28 days prior to the end of the 30 month residence.)
Maybe whoever prepared the visa can't add up?