Introducing Python
I love Python and Django, and use both to develop the our core webapps.
That said, it's hard to make a business case for switching at this point. Specifically:

Any new platform is risky compared to staying with the tried and true
You'll have the developer fragmentation you mentioned
It's far easier to find PHP programmers than python programmers

Moreover, as other posters have mention, if the issue is more with spaghetti code than PHP itself, there are plenty of nice PHP frameworks that could be used to refactor the code.
That said, if this developer is excited about python, stopping them outright is probably demoralizing. My suggestion would be to encourage them to develop in python, but not the mission critical parts of the app. Instead they could write some utility scripts, some small internal application that needs doing, etc.
In conclusion: I don't recommend switching from PHP, but I do recommend accommodating the developer's interest in some way at work.
The company I used to work with has two developers working fulltime, and a handful of freelancers. They're in the process of hiring a new lead developer to try to bring order and management to the development.
But, currently, one of the developers has seen the light of Django (the company has only developed in PHP to date) while the other developer is concerned that introducing a new language (Python) is a bad idea right now.
How should they approach introducing this new technology? Obviously with only one of the developers actually knowing Python, there will be no redundancy when that dev is away or leaves the company.
Should they bother to introduce Python, or should they look for PHP-only solutions until such a time when the team actually have more than one Pythonion? Without a team leader, the decisions are having to fall to them.