Cats meow mostly for one reason, to communicate with humans. Feral cats usually don't meow. Also, meowing is akin to a baby crying to its mother. Domesticated cats retain this behavior with us, since we're their caregivers.
What I'm hinting at is that your cat is most likely asking for something from humans. If they look at you while meowing, or are near you, it's a message for you. Otherwise, your cat might be "calling" for your friend.
Maybe your friend let the cat outside accidentally and they want to get back outside?
Cats can actually be very socially oriented and miss people when they're gone, despite what pop culture would have you believe. I personally never leave my cat alone for more than a few hours. If I have to leave town, I leave him at a family member's house. That's just me though.
Maybe your cat is worried you'll leave again?
My suggestion: find something to lure your cat away when they do that. Maybe a toy or something you know distracts them, not a treat / reward. Don't summon them specifically. We don't want your cat to think they'll get attention for doing this.
When they're not whining, maybe play with your cat / give them treats by the door. This way we'll replace whatever bad memories there might be with good ones.