I believe that is a bad idea for two reasons.
One, Evelyn is closer to Cadyn location wise, and thus it should be much quicker for her to reinforce him then it would be to traverse the mountains to reinforce Cerwyn.
Two, Cadyn is shown as "holding the line, but not winning" while Cerwyn is shown as handily winning against the Boarfolk even without our support. Now, this likely won't stay the case once Vasov returns, but with the reinforcements of both the Warriors and the refugees (which is a total of about two martial worth of fighting force) that should even the odds for her.