Revision of income tax return

Dear Expert Please guide me regarding the revision of ITR as one of my friends has forgot to show his capital gain from sale of property.he sold property in 2013-14 of Rs.25 Lac which was purchased in 2012-13 for Rs.20 Lac.but he filed his return for his salary only and did not show capital gain.now he got notice from department that property transaction was reported in AIR but assesses has not shown in ITR.So,please advise now what is the option.