First of all I would be very careful, and also backup your db.
ssh into your server
cd /var/discourse
./launcher enter app
rails c
u = User.find_by_email('email@test.com')
u.password='zKRR6vTZKvJAB84V9jdgeydS'
u.save!
ctrl+d to exit
I still think the user should figure out their email issues first though, so that they can just do a normal password reset, or change their email address for them.