Perhaps you should read our articles on what the Bible says about gay relationships and why is it wrong to be bi before considering what it means if someone in your friendship group, who is not a Christian, says that they are gay.
It may be tempting to ‘shove it under the carpet’ as a small thing. But it is important to remember that how we express our sexuality is a big deal; it gets to the core of our very selves – our mind, body and soul. This is why Paul writes, ‘Flee from sexual immorality (which giving in to homosexual temptation involves). Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.’ 
And consider this too. If the person is of your sex, then the declaration has a profound effect on how you relate them, since it opens up the possibility of sexual desires towards you.
Of course, Christians are called to be loving in all circumstances. But what does this mean in practice? The Bible helps us here. How we love people should be shaped by God’s word and not how the world understands love. In 1 Corinthians 13 the definition of Christian love includes these words: ‘It does not rejoice at wrongdoing, but rejoices with the truth.’ Remember that complete honesty is essential to any God-honouring relationship.
So it is loving to be concerned when a friend makes themselves vulnerable to sin and it is good to want to communicate this to them and preferably sooner rather later – it will only get harder with time. It will be important to explain what the Bible says about homosexual practice, and dealing with sexual temptation. You would also want to warn them of the potential damage – spiritually, emotionally and physically – that a wrong relationship can cause.
How you ‘speak the truth in love’  is challenging. As you think about how to do this, it is important to bring the situation before God in prayer, asking the Lord for your friend’s well-being as well as providing the strength for you to be faithful to God’s word and self-controlled in your emotional reaction to the situation. Remembering that ‘love is not arrogant or rude’ , you must raise the matter with a humble and gentle spirit. Practically, find a time when you are unlikely to be interrupted.
On a practical level, be careful what you affirm with your ‘likes’ on social media. The issue of attending celebrations also needs to be thought through. If the celebration is simply a birthday, like an 18th, and perhaps depending on the venue, you might attend. However, you would not be able to attend the celebration of a gay relationship  and you will need to explain to your friend why this is the case.
In all this your classmate may respect your honesty and may want to continue the friendship. They may even be willing to reconsider their position. Supremely you will be praying that they seek forgiveness from God and find a new identity in the Lord Jesus. On the other hand you may face rejection for your comments, which they may find offensive or even hateful, leading to a breakdown in the relationship. Even if you sense rejection, continue to show kindness to them whenever you can.
- 1 Corinthians 6:18 “Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.”
- Ephesians 4:15 “Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ…”
- 1 Corinthians 13:5 “or rude. It does not insist on its own way; it is not irritable or resentful…”
- 1 Timothy 5:22 “Do not be hasty in the laying on of hands, nor take part in the sins of others; keep yourself pure.”