While most of your Android contacts will show up as a green bubble in your Messages app, that’s not always the case. If your friend has another Apple product, like an iPad or Mac, it might default to their iMessages account, even though you’re trying to send them an SMS text. The message will end up on their Apple device, but not be sent to their phone.
To avoid this, you can tell your iPhone you want the message to go your friend’s phone number, and not the email associated with their Apple ID. Just start a new message in the Messages app, then type in your contact’s name. Before tapping their name when it pops up in the results, tap the arrow that appears to the right of it, then choose the correct phone number (which should show up in green).