Theravadan Buddhists don’t have sin; they do have bad karma. The question isn’t whether they feel guilty for what has caused their bad karma; the question is what they are going to do to make up for it. One could then bring up that there is someone who can take care of the problem of their bad karma. This would lead to Jesus’ substitution for the sins (bad karma) of the world.
Theravadan Buddhists do recognize that humanity has a problem. Humanity suffers because they desire. Christianity also believes that humanity has a problem that causes suffering. When one talks about suffering, they can then ask, “Can I tell you why I think people are like this?” and bring up man’s sinfulness. Secondly, Theravadan Buddhists recognize there is a solution, but it’s the work of the humans in achieving that salvation. Christianity also has a solution, but it’s the work of God. A Christian could recognize the difficulty of their solution in achieving that much good karma and how much that must bind them in life. It would then be possible to talk about the freedom of salvation in Christ’s work.
Halverson, Dean C. The Compact Guide to World Religions. Minneapolis: Bethany House Publishers, 1996: 54-67.
Various. Eerdmans’ Handbook to The World’s Religions. Grand Rapids, Michigan: William B. Eerdmans Publishing Company, 1994: 222-242.
World Religions Index. http://wri.leaderu.com/wri-table2/buddhism.html