If you plan to expand on the existing project or modify it, the developer should be using CodeIgniter. CodeIgniter is an application development framework unlike other application development frameworks in the sense that it comes with its own set of software development libraries. You could think of CodeIgniter is a basic foundation for building applications.
On the other hand, if you required a completely separate application, you could elect to use another framework but should understand the consequences of using multiple frameworks.
Generally, it makes sense to stick with the same PHP-language-specific framework as much as possible for all of your development needs. Later if you decide to develop your applications in house, your applications will be more consistent and code will be more reusable because they are based on the same framework. Also, your developers will have to undergo less of a learning curve working with a single framework.
What I would recommend is to prepare some sort of ‘Standards of Quality’ document to send out to potential bidders that identifies what you expect to receive when a project is returned for signoff. You can specify what framework to use, some kind of standard for writing code, delivery obligations, penalites for slow deliverables, etc.