I'm going to make the assumption that the rescue will not be some sort of immediate teleportation of the Runegraver and apprentice away from the bad situation.

One option could be some sort of carrier raven or other such bird, if trained, could carry a message from the Runegraver to his contact. This assumes that there will be a delay in the rescue, as it'll take time for the bird to fly and the contact to get to the Runegraver. There is also the possibility that the animal is intercepted before it reaches the destination, but that's really a decision that's up to you.

If the contact has the means, the Runegraver could have set this up in advance, and requested that the contact use something to scry on him at a specific time each day. There could be a specific signal or something previously communicated that the Runegraver would give at that time and if the contact saw it, they could react as they see fit. Though again (if you wanted to add some chance to the mix), the contact could be per-occupied and forget to scry or the Runegraver is too busy to give the signal.