[git] How do I push to GitHub under a different username?

If you use different windows user, your SSH key and git settings will be independent.

If this is not an option for you, then your friend should add your SSH key to her Github account.

Although, previous solution will keep you pushing as yourself, but it will allow you to push into her repo. If you don't want this and work in different folder on the same pc, you can setup username and email locally inside a folder with git by removing -g flag of the config command:

git config user.name her_username
git config user.email her_email

Alternatively, if you push over https protocol, Github will prompt for username/password every time (unless you use a password manager).