I would go with Kris48. Invest in an inflatable snorkeling vest, not all that expensive and compact. Start with a dip in the hotel pool, have your friend practice a little technique, keeping the snorkel tip up, how to blow it out, and how to not panic. Being in a circumstance where you can stand up if you start to get shaky is very important. After this, hit someplace like Ramon's pier where you can still stand up, but have something to actually look at. When your friend is comfortable enough that she (?) gets more interested in seeing things than concentrating on the next breath, then out to Hol Chan. It is a lot like diving, you have to be comfortable enough to handle things without panicking. If you have to concentrate on survival, you miss the point of doing it.