How to cure Frog's curse?

I don't think the requirements of this have changed on any of the platforms.

Did you specifically initiate the event with Frog in your party at the time?

If you have Frog in your party when you head to the North Cape, there will be conversation about the curse. Consequently, Frog will fight the battle on his own. If you win in this scenario, then it should result in the curse being broken for the ending.

Here are the steps according to this Wikia, I haven't done these endings in ages but all of this sounds correct.

Ending 1e: Revenge for Cyrus

Method: When Frog and Magus face each other on the North Cape, either fight and kill him or spare Magus' life

In the canon ending, Frog spares Magus' life, as killing him will not return Cyrus. Magus joins the group for the rest of the adventure, leaving at the end to search for his sister. However, if you choose to fight Magus in a one-on-one battle and defeat him, Magus' spell that transformed him into a half-human / half-fiend will wear off, but not until the end of the game.

After defeating Lavos, Magus' magic wears off, and Frog turns back into Glenn, who is a tall, muscular man. This is because he aged ten years while in his fiendish form. He bids farewell to the others, preparing to return to the Middle Ages. Lucca and Marle use the opportunity to remark on how handsome Glenn truly is, wishing that he could stay around longer.

This does not change the ending of the game.

For a complete list (of all the endings), check out the source below.

Source: http://chrono.wikia.com/wiki/List_of_Chrono_Trigger_endings