That's actually counterproductive, to be honest. Apple cider vinegar is acidic itself, and if you're already dealing with heartburn, you're just making the problem worse instead of solving it. The idea behind it is somehow that more acid "balances out" your stomach acid, but that's not how your stomach works. Your body regulates pH on its own - extra acid just irritates your esophagus more.
What actually helps: eat smaller meals, not right before bed, cut back on fatty and spicy food. If it's really happening regularly, you should see a doctor - sometimes there's something more serious going on, or it just needs the right treatment. Apple cider vinegar is placebo at best or makes things worse, and your friend means well, but I wouldn't listen to them on this one.