Sharing contacts and calendar events

Not sure if this is what you mean by complete reset, but this will wipe the drive and reset to factory settings and should not give you any problems.

Erase your Mac and reset it to factory settings - Apple Support

If that is not what was done, then Calendar and Contacts can still have accounts signed into them that are yours. Your friend would need to go to settings for each app:

• Contacts > Settings > Accounts
• Calendar > Settings > Accounts

They can then remove the accounts that belong to you.

I was thinking more if you had an Outlook account or even Gmail at Contacts > Settings > Accounts. When you set those ones up as a Mail account, you are also given the option to sync Contacts and Calendars. If you had one of those still set up on the old Mac, when you added a item on your calendar on the new Mac it would sync the contact to your old Mac using that Outlook or Email account.

Since you are showing only your iCloud account at that location, it would not be the issue. I also don't understand how the syncing is occurring since you reset the Mac to Factory Settings, which would erase the drive. The situation I described would only happen if you just signed out of your Apple ID and then a different Apple ID was signed in.

Hopefully we will get some input from other users who may have an idea.