We could get all fancy with Greek, Jewish wedding imagery, yada yada, but there’s a pretty quick argument that gets your main point across in a fairly strong way.
Protestants believe in imputed righteousness; that Jesus’ righteousness covers our sin, in a sense. Basically, God “declares” us clean. However, in Revelation 21:27 (NIV) states:
“Nothing impure will ever enter it, nor will anyone who does what is shameful or deceitful, but only those whose names are written in the Lamb’s book of life.”
It’s pretty clear from the above that you, as an individual, have to be perfect to enter, because impurity doesn’t enter, declared or otherwise. Another good thing to keep in mind is that Jesus condemned the Pharisees for being a cup that was clean on the outside, but dirty on the inside. Based off these concepts, we know that you have to be perfect to enter heaven. Now this is where you look your friend straight in the eye and ask: “Do you think you are as perfect as Jesus?” He will answer no. You ask “Then how are you getting into heaven, according to Revelation? If you’re saved, and you’re not perfect here, then it has to happen at some point after you die, or you’re not getting in.” Let him muse on that for a moment.