There isn’t no difference between day gambling and midnight gambling, the reason why that your friend said that he wins more at midnight, is basically because he’s on his winning streak.
Apart from being on his winning streak, he may be so much familiar with most leagues played at night. Some people don't play all leagues, they rather gamble on selected leagues where they are familiar with and often make winnings from.
Gambling at midnight is the same with day time, someone can loss as much as he did in daytime at night as well..
Losing in gambling can come at anytime, being in in the day or at night. I could remember a time I was favoured in gambling with Brasilian league played at mid-night. I often predict their draw and this worked for me for a while. After sometimes the story changes. Luck goes along with analysis in football betting.
Sometimes people tend to gambling at night before by that time they wouldn’t be busy that much with work stuff and all.
Some also gamble at night to reduce the stress of tracking selected matches every minutes or hours especially those that doesn't have mind for gambling. Some gambler often check there picks very minute to keep on track on how it's going, gambling at night may help to reduce this stress as they later sleep up. So leaving themselves with no choice than to only check the outcome of their gambling either it won or lose the following morning after being awake.