I received this statement from a good friend of mine concerning "evil"... "Evil is not a thing or force, but merely the absence or concealment of good. One need not "defeat" the evil in the world; one need only bring to light its inherent goodness."

How should I reply to this person?

Most likely your friend is not a Christian, for they would understand evil and how to combat it from what God says in His Word:

James 4:7  "Submit yourselves, then, to God. Resist the devil, and he will flee from you."

1Peter 3:1 "He must turn from evil and do good; he must seek peace and pursue it." 

1Peter 1:14 "As obedient children, do not conform to the evil desires you had when you lived in ignorance." 

James 1:21 "...Get rid of all moral filth and the evil that is so prevalent and humbly accept the word planted in you, which can save you." 

2Timothy 4:18 "The Lord will rescue me from every evil attack and will bring me safely to his heavenly kingdom. To him be glory for ever and ever. Amen."

2Timothy 2:22  "Flee the evil desires of youth, and pursue righteousness, faith, love and peace, along with those who call on the Lord out of a pure heart."