If that developer hasn't been producing and was a bad choice for the project, then there isn't much more he/she can possibly contribute.
By bringing in another developer to take over the project, that developer is going to need to look through the entire project and make a determination of whether or not there is any value to any code written so far.
Ramp Up Period for New Developer
You'll need a developer who is capable of reading code, not just writing code. Many developers' first instinct is to toss out any code and just rewrite from scratch. That's not always the best decision, and you'll need to make sure the developer taking over knows the difference between something that can be brought to market with determination and elbow grease and something that is not salvageable.
Project Manager Must Understand Learning Curves Exist
The project manager will need to understand that the developer taking over can't just jump in and immediately show results. There will be a learning curve, and it will take time for the developer to assess the current state of the project.
Goals of Project May Change Based on Stakeholder Communications
Additionally, since it's an important project that is now a delayed project, the goals of the project may have changed. For instance, if time to market and timely delivery is critical, the PM will need to make sure the developer understands that -- if at all possible -- he or she absolutely must work with what the previous developer left. That means that things might not be perfect. The decisions here largely depend on the client/stakeholders and what their needs are.
In summary, the project can be salvaged, in most cases, but it takes having the right people take over, having a clear understanding of what the modified goals are of the project, and maintaining excellent communication with stakeholders.