I am fairly sure of the answer to this question, but I am posting here because the other party involved is still not convinced. Last night we had a game going between me and my two friends who we'll just call John and Jane. Jane had acted first and knelt most of her board. John was up next and decided to declare a power challenge against Jane since most of her board was knelt. He knelt Robert Baratheon (Core) and said that he would "do a power challenge" against Jane.
At this point I said that I needed to look at something in the rule book. As soon as I said this John said "unless there is some kind of effect that's going to happen." When I declared my intention to redirect his power challenge towards me with the crown regent title (so that fat bob would not claim 2 power) he attempted to "un-declare" the challenge and argued that it was legal for him to "re-declare" his attackers because of his "unless" statement; his intention obviously being to attack with more characters so that he could now win the challenge against my board. I vehemently disagreed and Jane agreed that John's actions were not legal, but he persisted. What should happen here?