If they are already on slalom gear they have good techniek, so learning to foil will go pretty fast i think.
I did the same and my first foil was a freeride foil. After 4days on the water (average 3 to 4 hours/ day) i could already fly 500m easely. I learned it in 8 to 10kn wind.
After 1year i switched to full carbon starboard race foil because i wonted more speed. Am also on a lake so wind is most of the time around 7 to 12kn.
So going to full racefoil i took 115 fuse, 1000wing and 800wing. 255 and 255-2 backwing.
With 1000 wing its also super stable but more fun because a lot faster then before.
You can start with a freeride foil but they will move really fast to some slalom /racefoi gear.
