Point is that FR have been using GDS for over a year, hence could understand if it was on day 1 but a year plus is not even close.
Problem for Hertz will be that it will lose a substantial portion of business who booked via Ryanair, now they will book everybody, Hertz will of course pick up some of this business anyway maybe at a better price.
However I doubt that this will compensate as simply their fleet buying decisions will already have been made and losing a substantial element of business will be a struggle.
Will be interesting for some of the Hertz Franchise holders who probably will have a claim against Franchise owner. If they acted to damage their business by cancelling a contract with a major business provider.
We don't know if they just stopped or they discussed it with Ryanair prior to the announcement. There is a lot more to this I expect, will be an interesting case.