Being that I am a salaried employee, I would be getting paid while taking the photos anyways so what I charge would be for any processing time, etc.
If you're taking these and processing them all on company time, then it's assigned work regardless of your job description. In that case, you turn in an expense voucher.
If you are expected to do this as though a separate job on your own time--and the offer for pay suggests that's what's in mind, then you approach it as a separate job.
Don't do this halfway between the two positions. It's it's either on their time or your time, and if it's on your time, you do it in a fully businesslike manner.
If you like where you work and you like the people you work with, why would you charge them? Maybe charge them any costs of processing, and even then, only if they ask for a 16x20 that will set you back more than $50 or so. Otherwise, I would even cover the expense of processing 4x6s or digital images.
Just because they offered to pay you doesn't mean you should. Sometimes making a buck will cost you two in the long run.
I'm sorry, but this is the "Business of Photography" forum. We're not talking about taking happy snaps of fellow employees at the company picnic for the internal website. We're dealing with a business for a product product to be used commercially.
To think that charging for work would "cost" him anything job wise is insane. Not even earn as much as he'd get working at his desk for the same amount of time?