Friends.

So im getting quite upset because my SO has a friend who comes over every day and stays for hours. Not like 2 more like 5-7 hours. He always just randomly comes by with put being invited. My SO never tells him anything and ive complained a few times to my SO but hes never does anything. Am i upset over nothing, or is he over staying his welcome. It bothers me alot and makes me feel like my SO other values his friends more. He even invited himself over on our anniversary and stayed the whole day till my SO went to work.