Generally, acceptance is a good idea. Since that person came out to you, it shows that they trust you and see you as a great friend or family member or co-worker, whichever it may be, who you will support no matter who they are. So, because of the value of trust they feel towards you to tell you their gender/sexuality, letting them know that you support them is a good idea- it helps them feel more freedom and helps you accept a little more that they are LGBTQ+. You can remind them that you’ll be there and maybe give them a hug or something.
If you let them know you accept and love them for who they are, you will make a difference for them. Make sure they know you're there to support them and they can talk to you about anything they want to share with someone. Never push them to do things they don't feel ready for - like coming out to their family or anyone else - and make sure theh feel loved. I'm sure you can be an amazing support for them!
Being LGBTQ is something that many people may experience ridicule for. Its important to make a loved one feel safe if they choose to confide in you with this information, and let them know that you take them and their identity seriously. Offer them your time, and listen to what they have to say about it. If you're unsure about how you feel, or have trouble understanding what they're trying to convey, ask them to explain further. You may voice concerns, but be sure to do it in a respectful and understanding manner. Being kind and rational is very important when speaking about such a sensitive topic, especially if this person feels you're important enough to them to share it with. Making them feel safe and loved it definitely key.