This could be a number of things. It could be the server or the client.
Easiest to check would be the persons computer. Is there any background applications running that are consuming bandwidth? Youtube, torrent, cloud based programs, updates?
Is your friend in an apartment using a wireless connection? May have to change channels.
Is someone else using the internet? Someone using a streaming service or something else using a lot of bandwidth?
Could try resetting the router.
Without any information I am just throwing out general things.
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