Got to this thread a bit late I'm afraid.
Candid shots are often much better than staged shots if you don't have experience. So you may find you have by luck achieved the best solution.
With regards to the weather and to quote a fantastic photographer whose landscape training course I attempt to go on every year in the Lake District:
'There is no such thing as bad weather, just different opportunities'
John Gravett - Lakeland Photographic Holidays
http://www.lakelandphotohols.com/
You would have done well with a window as suggested in almost any kind of light. Taking any photo in RAW is a must and it would be easy to adjust colour problems/balance afterwards in something like Lightroom or Photoshop. Or of course software probably supplied with the camera. Personally I create my colour balance using an ExpoDisc. This allows me to get the 'correct' colour balance and thereby reducing post production work on the computer. It also means I don't have to 'remember' what the colour was like a couple of weeks later when I am going through maybe 1 or 2,000 photos!
As to flash, this is one of a portrait photographers greatest tools. But you need to obviously use it correctly. I would have suggested that you find the setting for fill flash. Check it out as this is a superb way of getting a shot of someone but keeping a little light in their eyes/face. So you then end up with the classic catchlight that photographers strive for. If you imagine taking a photo of someone in sunlight and they are wearing a hat. It can be very hard to get light in the face without 'blowing' out other parts of the photo. A fill flash will do this for you and allow the rest of the scene to be perfectly lit.
I shot a wedding during Christmas and New Year last year. It was very wet so getting the Bride and guests outside was impossible. The smart Hotel had no large window areas. Having arrived two days early I had already surveyed the inside of the Hotel and found the reception was the best area. I used one flash set to fill and was able to light groups of up to about 10 people at a time. It worked well. During the ceremony in a dreadfully lit but 'pretty' room, I used one on camera flash for fill and one off camera portable flash as the main light. Again it worked well. During the morning as the Bride and Bridesmaids were getting ready, in the dark under the worst fluorescent lights I have ever seen, I stood in the bath and took many hundreds of photos as the makeup was put on. I had one flash for fill and bounced it off the ceiling. I confess that even using my ExpoDisc I was not happy with the colour balance. The trick is then to set the actual Kelvin level. This means when you come to post production, you can determine the exact colour balance needed then apply this to all of the photos taken at that specific session.
There is so much to learn in portrait photography that I think you may, as mentioned, have the best shots by taking your candids. Rather than a book, why don't you create a 'movie' of the photos overlaid with your mates favourite music. If you own a Mac computer this can be done easily with the supplied software and you can even add chapters. So you may have some photos of him in the past which you can also use. I do this for almost every photoshoot. It is easy and I never fail to get tears from the recipients.
I should also point out that I own the following:
Nikon D3S
Nikon D3X
With most of Nikon's fast lenses including probably the best portrait lens they make the 85mm 1.4G
Studio lighting & equipment for on site and studio lighting equipment for field work.
And yes it is a very big hobby of mine!
Best regards
Chris