Ok here is the argument you use ...
She is drawing a parallel between the two because she sees them as the same. All you need to do is show the differences between the two in your argument.
1. A committed relationship is consensual where as molestation of children is an older person taking advantage of a younger person who isn't mentally developed enough to understand what is happening. It is someone who knows better inflicting possible harm against an innocent child. An agreed upon relationship between adults is a relationship of adults who have an equal understanding of the situation. A molested child can be mentally scared as a result of the situation.
2. Homosexuality was not God's original plan, but the basic component is still in gay relationships as well as straight relationships ... LOVE. The relationship is not perfect, but it is not evil. Although it is not a perfect relationship the way God originally intened, the element of true love can still be found. Pedophilia on the other hand is evil because it can harm children who aren't mature enought to make those choices, again, homosexuality doesn't cause harm to either party, molesting children hurts the children for the rest of their life.
3. Homosexuality is not a choice. It is a sexual difference. One can choose not to molest children. In fact, pedophilia is more about taking advantage of kids who are innocent, for the persons own personal satisfaction at the expense of the child's welfare.
Great harm and mental distruction exsists in one (evil, sin), an alternative way of love in a same sex relationship (not perfect, sin, but has the basic element of god.).
Sin is not always evil, but evil is always sin.
Example:
Evil Sin = A man abducting, raping, and beating up a woman. He hurt someone else for his own pleasure when he know someone else would be hurt, and felt no remorse for his bad behavior.
Imperfect Sin = A woman drinking a diet pepsi. A man smoking cigaretts. A woman screaming at her children, but later feeling bad and making reparations to her kids.
Evil sin is doing something that hurts someone else in a way where you benefit and have no remorse for doing so.
Imperfect sin is doing something that isn't God's origional plan and missing the mark of his perfection (a catagory we all fall into on and off at some times in our lives). Not being perfect and getting away from God is sin, but it is not sin that hurts anyone else. Yes, drinking diet pepsi is a sin because God did not create it and Jesus says that you body is your temple. Smoking cigaretts is not of God. He would never endorse anything that would hurt the body and cause disease so smoking commercial cigaretts is imperfect sin!
Point out to your friend that eating shell fish is a sin! It says so in the bible clearly. If she drinks someting that is not naturally found in nature by God, then she is comitting a sin! But let her know that it isn't evil.
Good Luck.