I can see several sides to this topic. The OP has a lack of experience, ZERO headshots in their portfolio, doesn't have a flash, is asking about using the pop up flash, won't be able to balance color temp between available, flash, fluorescent, and works for 'the boss', no experience directing people, and on and on. The OP is clearly not ready to shoot this.
But should the OP do it? Maybe. If there is no down side other than some pictures that don't work, and some people lost some productive time having fun trying to shoot pictures, then why not! Its excellent practice in a real world situation. So it depends on what the outcome would be if the OP doesn't deliver on whether av3nger should or shouldn't.
I would also recommend available light if there are ample windows with diffused daylight, or good interior lighting. That can easily be checked by av3nger bringing his camera to work for a day and taking some test shots to check the lighting. If av3nger is inexperienced in shooting portraits, why throw another wrench in the works by adding flash?
Shooting available light is easier for someone new, and the results can be very pleasing. I would recommend that if av3nger tests the lighting in the office and can get decent results, available lighting with a reflector or two would be the way to go. With a little practice on others before the shoot, it could go very well.
On the other hand, having the OP take a flash they are not familiar with, and try to use that to get a nice portrait shot could be far more difficult. There will be issues balancing the flash to avoid the black tunnel effect, color balance issues between the subject/flash and background/fluorescents/tungsten, plenty of settings that could get mis-set causing lots of frustration and troubleshooting time, and so on.
Ideally, flash is the way to go, or even better balancing flash and ambient together. But for someone new to portraits and using flash, natural light with a reflector is the option I feel would generate the best results in this situation.




