They will have to apply for a visitor visa

Your fiance is the one to prepare documents to prove to visa officers that they only plan to visit, and not to move to the US.

If it is only for a visit, it is your fiance that has to worry about what to prepare. But if your intention is for them to come and live with you in the U.S., you can sponsor them for a K1 fiance visa, but you must be a citizen.