First, given most Protestants’ attention to the Bible, you can provide John 20:21-23 as an answer from a purely biblical standpoint. In fact, here’s a useful link containing many scriptural verses on sins and confession, if you ever feel the need to share it with your colleague or anyone else: scripturecatholic.com/confession.html
Second, as a “non-biblical” answer, you could say that although only God can forgive sins, there’s no contradiction with the fact that He applies forgiveness through a priest. Also, you can add that confession allows us not only to make sure that we confess all of our sins but that the priest, as someone who is usually a learned man of God and guided by the Holy Spirit, will make it possible for us to be forgiven/absolved.
Provide these answers as a loving witness to the truth (which can be hard, I know) and hopefully they’ll think about it, at least. Good luck.