Based on last year's summer you wouldn't get out much on that gear. I usually used a 7.4 sail and a bigger board. Hopefully this summer is a bit sunnier and windier than last summer.
I only had a 6.6/100l and went out 4 weeks out of 5 the summer before. But I'm quite a light rider. Even though last summer might have been unusually not windy, I'd still agree with you that he should go for a 7.0/7.5 just to be sure...
And then a 5.5 for the big days. But then you need two boards....