Your friend is wrong. You don’t have to supplement protein right after a workout. As long as you get enough protein over the day, your body will be able to use it. Source: https://jissn.biomedcentral.com/articles/10.1186/s12970-017-0177-8
Edited to add key quote from that paper: “Total protein and calorie intake appears to be the most important consideration when it comes to promoting positive adaptations to resistance training, and the impact of timing strategies (immediately before or immediately after) to heighten these adaptations in non-athletic populations appears to be minimal.”
And yes to your second question — IF can have beneficial effects even when you are trying to maintain weight, not lose. It keeps your insulin levels lower during the fasting period, which can improve your body’s overall insulin sensitivity — which in turn will reduce your risk of various diseases, and may potentially make it easier to maintain weight in the long term.
I recommend reading Dr. Jason Fung’s The Obesity Code if you want to learn a lot more about insulin, IF, and health. (Despite the clickbaity title, it’s not just about obesity.)