The safety pilot is the pilot who performs the required functions such as lookout, monitoring the pilot flying undert the hood and if necessary takes control of the aircraft should it be required eg pilot flying suffers disorientation or is about to fly into a hill.
Define the job of a safety pilot?
If the flying pilot is IRed, but behind screens or foggles, he wants the safety pilot to keep a look out. He may want him to perform other functions, but he may not. If he does not, it would seem pretty pointless for the safety pilot to have to hold a IRing before the flying pilot entered IMC.