I need help processing this and doing the right thing because of or despite my Christianity.

I don’t see what would change. Aslong as they don’t come onto you they’re still a friend.

Love is love smiles are smiles friends are friends no whatever their taste

Do NOT hurt them at all. They trusted you enough to come out to you. Trying to "fix" them will do irreversible psychological damage and cause massive trust issues with them.

They are still human. Broken as each human is. Deserving of basic human decency and compassion. If your BFF is a follower of Christ, there is an issue if they act on their attraction. Let's take the stricture against fornication. That is a problem with God.

His dad is a pastor

If your dad should disown him or treat him with l will then he has broken the very basis your religion in that he has decided to remove the concept of kindness and common decency in relation to his own son.
Should your best friend act on the fact they are gay this is little issue as an all loving God should take little issue with it, as long as he has done good deeds and shown good will to his fellow man then there is little issue.
As for fortification all sin is equal under God as I take it, somehow anyway apparently sex is equivelant to murder but who am I to judge.
Resident humanist out.

Oh and another thing if he does get kicked out or suffers abuse try to reach lgbt services where possible

I too, am a sinner. I too, have fallen short of God's laws. God is not simply "all loving". He is also infinitely just. The ultimate judge that wrote every law that He judges us by. He doesn't grade us on a scale (A, B, C, etc). It is either Perfect or Fail. In a human court, if someone is found guilty of breaking a law, they are punished in some way. Good deeds, in an equal justice for all human system of justice can mitigate some offenses this is true. God's system has no mitigating circumstances to breaking His laws. You stated correctly that, to God, sexual sin, theft, murder, and other sins are all factors that will keep each of us from His presence for eternity. If we break one rule it is the same as breaking them all. "The wages of sin (a singular sin) is death (eternal separation from God)". We may each choose to follow and trust in God or to hate or even ignore Him. We don't get to choose His verdict. In my first comment I feel I was clear that this person should be treated with love and compassion. I would not be compassionate telling them or anyone else that this sin or any other would be okay. That's not compassion, that's complicit. I am thankful that by placing my trust in Jesus that He gives me grace, His mercy that none of us can earn. Believe it or not. Heaven and hell are real destinations. Our time here on Earth is miniscule compared to the eternity that awaits each of us. Our choices here will determine our final destination. May you live long enough to make the right choice.😌

I have a correction:He is bi.

Fornication is defined by God as any sex outside that between a husband and his wife. Gay, Bi, Adultery.. all a sin.

At the end of the day just remember to be kind and decent to your friend.
Do not attempt to make them change their sexuality even though I doubt you would think that.
Should their father abuse them remove and get help preferably from known lgbt charity's.