They haven't written it down to nothing, its clearly still worth ~€150 million at current share prices, but they have conceded they'll never get their money back. While they won't get offered anything close to what they paid for their stake, they'd be stupid not to try maximise the value from any sale of their 29% stake.
Incorrect as they have reduced massively the value of the investment in the accounts, they are legally required to do by Stock Exchange rules and show the loss in their reported profit figures.
Once they reduce it then it has little impact until disposed of or the share price falls below the value held in the books in which case they would show a further loss.
Anybody who buys 50% of Aer Lingus will have to buy FR's stake at the same price at a minimum, that is if they wish to sell, legally they do not have to and nothing can force them to.