Aruba was my first time snorkeling back in 1998(?). We went on the Jolly Pirates and had a great time. I would recommend a catamaran though -easier to get one and off. I snorkel all the time and I can't even swim!!
My swimming pool with a 7/7 or in
my boat with a fishing rod looking in at Malmok.
I have a family member who got the first lessons in the pool at Aqua Windies and then they too him on a private snorkel trip in the ocean after that.
More expensive but definitely the best way to start if she is a little intimidated by it.
If she is a good swimmer and not at all afraid of the water I would say go nice and easily from the shore yourself in 3 feets of water. Baby beach is a great place for a beginner.
I know they do on the Jolly Pirates...i was on there in January and they had I think it was a half hour long instruction on the way to the first snorkel spot...I heard from some that were listening in, it was pretty good and very informative