a)
Kufuo is a friend of the writer's. He's a cocoa grower in Ghana and does work for Fairtrade.
b)
- 've worked / 've been working ('ve been working emphasises the length of time)
- 've visited
- 've lived / 've been living ('ve been living here slightly emphasises the idea that it's temporary)
- 've met
- 've known
- 's grown / 's been growing ('s been growing emphasises the length of time)
- has followed/has been following (has been following emphasises the length of time)
- 's recruited
- 've doubled
- has become