You'll have never seen this person before in your life, or if you have they will be socially awkward and not able to hold conversation. What these people are exceptionally good at is not knowing what they want, doing circuit after circuit of the shop picking up items and putting them back in a never ending circle of futility. On the occasion they do leave they will be replaced by another almost in the same instant. By this point you'll have pre-written a "back in 20 minutes" sign for if you ever manage to escape - but deep down you know you'll never get to use it.
Sometimes it might seem like a relative shopping for others is an anonymous browser, they're not - they're just confused about what to get. Go help them, don't mistake bad customer service for a bad or annoying customer.