I guess my friend’s landlord might have to use the professional exterminator to deal with mouse problem...
Perhaps, depending on the structure. Modern single-family homes are designed to resist rodents, so entry holes can usually be determined with a careful inspection. Eaves, roof flashings, garages, basements, and any sort of cracks or gaps are the most common points of entry. Or, it could be a single mouse that scurried in at night unseen when a door was opened.