When are you supposed to receive Schedule K-1 & what has to be included on tax return?

Hi!  I have a family member that I assist with filing his tax return.  Due to a particular investment that he's involved in, he receives a Schedule K-1.  This has been the case for about 2 or 3 years so far.  However, he always receives it months after the April 15th deadline for filing your tax return.  Most tax forms are required to delivered by a certain date prior to the tax deadline.  Is it different for K-1s?  Does this mean an amended return has to be completed every year?  How do you know what information may need to be included on your tax return or if it's just informational?  Thank you!