how do you want to split it Salutor?
It's tricky, cannon is not clear. I dug through RoP:M to see if it mentioned it, and found nothing. Forums tell us Can a being of Might voluntarily lower its magic resistance?, but that is dealing with fae rather than a sentient magical beast. Summary of that thread is that some folks think the cannon implies they cannot, others opt to say that fae could so that they could benefit from the magic and spells of their fellow fae.
IMHO it makes sense to me that a creature could choose to be affected, so that it can be assisted by it's fellow creatures; but that choice would be rare when dealing with humans and Magi. I suggest that Alcimus probably does not trust Fray well enough to even allow it if he could, although given the circumstances perhaps he would have (your call Salutor). I'm certain that Fray would not seek to anger either Alcimus or Gregorius by trying to penetrate through Alcimus's MR.
We retcon the story so that Alcimus stayed at the cavern entrance?
cheers IBT