Is it obligatory to return the greeting to a worshiper who greeted from his right and left?

It is not obligatory for the one to the right or left of a worshipper to return the greeting to him, whether he is praying or not praying. If he is not praying, he is not obligated to return the greeting, though it would be good if he did. As for one who is praying, he is not required to return the greeting while in prayer, nor if someone greets him after he finishes. However, if he does return the greeting, there is no harm, due to the general command to return greetings.