I know someone who has a taser, thery want to know how to legally be able to use it in their state in rhode island.?

They can't. Under any circumstances.
The sale and possession of tasers is banned in Rhode Island. What is unclear here for you?

The same laws for firearms apply to tasers.

Yes

I doubt they are legal for private use. Contact your local law enforcement.

Banned for all private use

So check with lawyer in Rhode Island. some concealed Carry Permits in some states have a class of 'Firearms' and a separate class of 'non firearms' that may specify 'electronic' devices. Locally the concealed weapons license law has sections on 'electronic' , Gas, sonic weapons or defensive devices with notes on which ones require the permit, a few that do not require permit. Used in fixed place of business as the Illinois term was written, some weapons that would require permit on street were legal in home or fixed place of business. Does the Tazer owner have a small business that might be target for strong arm robber? Home subject to druggies burglars? Does Rhode Island have a police and military only weapons list? Security/detective tools allowed for politicians bodyguards list?

Use it FOR WHAT?

They were designed to repel Jehovah's Witnesses and Mormons on bikes. You have to warn them first.