I'd like to take a username from a dead account that has no friends, no gamerscore, an Xbox...

You can use t, but you will be given a suffix since the tag is already in use on the servers.

The username I'm looking to take is 'Fnqi'

There is no way to request a gamertag be free up.

As of 2018 if a user doesnt sign into their xbox profile within 5 years then their gamertag would be freed up. If they dont sign into an associated Microsoft (such as outlook, office 365, etc) service within 2 years then that would also free up their gamertag.

So basically all it would take for their gamertag to never be reset would be to login to their Microsoft account to check their email once every 2 years.

To add, if there are any others with the same name (with a hashtag and numbers at the end) then even if the original gamertag without the hashtag is freed up youd still have a hashtag and numbers at the end of yours unless all the people who have the same name change theirs to something else as well.

Just because a Xbox account has 0 gamerscore and 0 friends, doesn't mean the account is not active somewhere on the microsoft network. If it is still being used, which it seems it might be then the username of the account cannot be given to someone else.

There's plenty of clubs on Xbox Live made for the sold purpose of finding new friends. I'd also recommend checking out the Xbox Ambassadors program, its also a good place to find new friends and take on engaging activities.

You can add me if me you like, too... My gamertag is my username on here too.

Sorry, but you can not (nobody can) undo what that 'friend' has done. it may take some time, but your gamerscore will likely be reset to 0 and will be labelled as a cheater. You won't be able to re earn any of those achievements with the account