The thing is it isn't really possible to tell you exactly what settings you need to use we can only give you general advice.
Flash for portraiture is a whole subject by itself, I would probably lean towards using ambient lighting as much as possible for this. What is the unit like in terms of ambient light? I would try to get some pictures done using window light maybe? If you haven't got a reflector get one - they aren't very expensive at all. You could even get a sheet of stiff card and stick some shiny foil on for a homemade reflector. Have an assistant hold it just out of shot reflecting light into the face of your subject but be careful not to cause glare, nothing worse than people sqinting in portraits!
I would watch your white balance very carefully - offices tend to use lots of strip lights which can cause ugly colour casts if you don't know what you're doing so be careful. You can use custom white balance, or turn the lights off and use just the window light. If you do use the flash you really need to colour correct it with a gel and then set white balance accordingly. Also, if you use your flash maybe bounce it instead of using it direct - make sure the surface you use to bounce it is a white or near white, as it will pick up colour and again cause colour casts.
Try to use the lowest ISO you can get away with, and you also want the fastest shutter speed you can to avoid camera shake blurry shots, however in low ambient conditions you might struggle so use that tripod you mentioned and you'll just have to get your subjects to remain still. That said, you still wouldn't get sharp shots at very low shutterspeeds, you'll just have to experiment and see whats working best. Use your fastest lens and open that aperture up but watch out for narrow depth of field. If you shoot 'wide open' you want to be focussing on the nearest eye (eyes should be sharp). It's a balancing act between what looks best composition wise and getting everything that needs to be sharp within that narrow depth of field. Above all, take many many pictures - don't just take one shot of each person and move on, you want to take loads and loads, fill your card(s) up, it doesn't cost anything. The more shots you have, the more keepers you'll have.
As for the charge/don't charge thing - you're happy to do it for free, you're helping your mate. It's not like its costing you anything to do, so why not? Use it as experience, if you decide its something you'd like to do more of then you can always go down that route later. You're mate will 'owe you one' and thats not a bad thing is it?
but i need help before i go and make a fool of my self 
Thats my two pence worth lol.
