Yes, this red envelope is a gift. When I got married, a friend gave it to me five days in advance because I could not go because I was pregnant. This is all understandable. Good friends don't care about this. Just give it in advance, because if you get married on the right day the next day, you will be very busy and easy to lose things. Give her a red envelope if you help in advance.
Let me give you a suggestion. When giving a wedding red envelope, buy a small red envelope package, put money inside, and write your own signature if you write a blessing outside. This is more sincere. I keep all kinds of red envelopes my friends gave me.