Billboards are often printed with an apparently ridiculously low resolution if you only consider their absolute size. If you however also think about the typical viewing distance and how small they appear at that distance, it all makes sense. The resolution of a typical human eye is not high enough to make it necessary to print billboards in a higher quality.
Here is a recent YouTube video with a more precise look at an actual billboard. According to that video, 2MP seems to be enough and it actually sounds reasonable: https://www.youtube.com/watch?v=UCaZt5ndRW0
So if you only count pixels, you would cope with a 15 year old camera from the dawn of the digital photography era, the cheapest possible lens, forget the tripod and use highly compressed JPEG. There is however much more behind decent marketing photography than just the technical parameters of the equipment. Do you as a photographer have what it takes to make a highbrow presentation of your friend by taking care of composition, posture and lighting of the image?