Is it obligatory to say "Yarhamuk Allah" (may Allah have mercy on you) to someone who sneezes in another room and cannot be seen?

If a sister sneezes and praises Allah, it is legislated to say to her "Yarhamukallah (may Allah have mercy on you)," even if she is in another room, because of the Prophet's (peace be upon him) saying: "Allah loves sneezing and dislikes yawning. If one of you sneezes and praises Allah, it is a duty upon every Muslim who hears him to say Yarhamukallah to him."