Hi Pacs and Hyper,
I think there may be a little more here than simply width and fin size.
I have the Futura 133 and have had it out in quite a bit of wind.
Yes, it's a little wide for 20 knots, but if I tune it with the mast foot, and use a little smaller fin (42 cm) or weed fin (Tangent Dynamics Reaper or Lessacher Duo 32 cm) it works pretty well with a 5.6 m2 Sailworks Hucker.
So, if your friend really wants the lower wind performance, the Futura 133 (76 cm wide/50 cm 1' off) will give significantly better results in early planing (and you can add a 52 cm fin to enhance it even some more) over the Futura 122 (71.5 cm/ 46.7 cm 1' off).
It guess it depends on how much of the time the wind is 16 knots and higher vs how much time it's 14 knots and lower.
If mostly higher than 15 knots, and your friend doesn't sail much in < 14 knots then the Futura 122 looks pretty good.
If, on the other hand, the wind is less often over 15 knots, and your friend wants to have some performance in 12 knots with an 8.0 m2 or larger rig, then the Futura 133 is the way to go.
A bigger fin won't make up for 5 cm extra width.
If you are happy on your Futura 155 with a 6.5 m2 your friend should be equally happy with the Futura 133 in the same conditions.
Hope this helps,