Can I Use Steam on Someone Else's PC Without Sharing Files?

If you and user 1 are using the same Steam account, then yes, that's going to merge your gaming profiles. Ideally, you should create your own Steam account and log in on the borrowed PC. If you want to access user 1's game library, you can set up Family Sharing. Just remember to log out when you're done. If you're only concerned about Windows accounts, Steam keeps game files separate, so user 1 shouldn't see your files unless you back them up somewhere they can access, like OneDrive.

You've got a valid concern! If you're logged into the same Steam account, your profiles will be shared, but you can create a separate Windows user account on the PC to keep files isolated. As for your buddy not owning a game, no, he shouldn't be able to see any game files from Escape from Tarkov unless he has access to your Steam account.

It sounds like you have everything set right! Just make sure you use your own Steam account to play. If you're still worried about it, check your Steam folder settings; they should remain separate for different accounts. Keeping an eye on folder access can help too!