Rebound relationships are usually not good for either person involved but they happen all the time. Everyone knows rebounds are bad unless they are involved in one It is also unfair to just judge just one person. A relationship takes two people so the 'new partner' is responsible too.
Every situation is unique though. Some people start relationships with others while still 'happily' married, some when the marriage starts to go downhill, some immediately when the marriage ends and others wait a long time after the end. It is hard to say how much a person has detached from their previous relationship just based on the length of time. As Andrew said, the ability to GAL without needing a romantic partner often plays a big part.
It is also unfair to just judge just one person. A relationship takes two people so the 'new partner' is responsible too. 