As this is a 'friend of a friend' story I would like a little more info.
The young lady may be classed as a resident, rather than a tourist, and it is not uncommon for students to sell items that they bring into the islands when they leave. The Customs Officers are aware of this, and may have charged import duties for this reason. I do not live in Grenada, but I have lived in St. Vincent and the Grenadines for over 10 years, and the Customs policies in most of the Organization of Eastern Caribbean States (OECS) are very similar.
Please note that holiday makers are allowed to bring their own gear without charge, as they are not likely to sell their gear prior to returning home, and registered businesses have concessions that they can apply for to assist with the cost of import duties. Most OECS countries are very dependent on the collection of duties as revenue.
If the young lady keeps the revenue receipt, she should have re-course to reclaim the duties when she leaves the island with her dive gear.