I wouldn't take it now. I would finish your "assignment" and then take it up afterwards. My experience is that when there is a project on-going the search for people is continuous. Tell your colleague you're very interested and when you will next be available. As you say if leaving damages the project your PM will make complaints and might scupper the move anyway.
The other thing to bear in mind, is that as a contractor every assignment is temporary, you're not moving into a new permanent job for several years, taking these risks for what is essentially a project lasting no more than a few months isn't worth damaging a relationship with another client.
The other thing to bear in mind, is that as a contractor every assignment is temporary, you're not moving into a new permanent job for several years, taking these risks for what is essentially a project lasting no more than a few months isn't worth damaging a relationship with another client.
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