I think the implication is that lynette has some strange magic nullification power, which she can transfer to others by touch, since the prince has likely been under the witch's spell all this time, what makes lynette special is both her power and the fact she could bear his pain which made him fall in love with her, basically the witch cannot use magic on lynette, and by proxy lynette can dispel any magic she uses, so for the prince to defeat the witch, they "both" had to strike her down, as the witch's magic still effects the prince and the guards, this is also likely why she was never around when lynette was around, she was aware of what she could do to her.