A single mouse may stray into a home and be quickly eliminated, but that ought not to be the default assumption. Any appearance of a mouse or signs of a mouse (droppings, gnawing, spoiled food) should produce a thorough effort to locate and repair any point of entry.
I guess my friend’s landlord might have to use the professional exterminator to deal with mouse problem...
You need to find out where they get in, there is an endless supply of vermin outside. I like traps, poison may leave you with a stinky dead mouse hidden somewhere in the wall. They can enter through the smallest of openings, around pipes, under doors etc.