Dont ever judge anyone according to their past, however it is important that you know about their past- its a compliment to their strength in character and what they have become or maybe in a way that they still may be struggling and as a friend you can help them out. Love demands perfection in the object of its love. However that perfection is not primarily achieved through the demands made upon the one who is loved but through the sacrafice of the one who loves. To love does not stop with who or what the "loved-one" is but seeks to produce in the one "loved" what he may become. When we approach our friends, family, significant others in this way, it becomes easy to forgive and move on in a way that will empower and avoid bitterness and resentment that accompanies betrayal.