1) Quando a construção for "prepare" + substantivo, utilize prepare for
- Have you prepared for your interview
- This course aims to prepare
students for middle and senior managerial positions
- Traders began to prepare for the annual surge
of Christmas shoppers.
- You need to prepare
yourself for a long wait
2) Quando a construção for "prepare" + verb (ou seja, "prepare" + outra ação), utilize prepare to
[prepare + infinitive]
- prepare to do something: Medical teams are preparing to leave
for Mombasa to treat survivors.
- Are the players mentally and physically prepared to play a tough game?
- It almost seems as if she is preparing to die
Por isso, acredito que na frase proposta seria melhor substituir "for leaving" para "to leave".
Espero ter ajudado!
Referências: Cambridge Advanced Learner's Dictionaryhttp://www.macmillandictionary.com/dict ... sh/prepare