What can someone access on my device if they have set up iCloud?
You can go to iCloud.com and login to check what can be seen. (Or don't you have access yourself to it?) Why did he login your iPhone? Couldn't you ask him to log out?

To summarize iCloud.com and what can be seen there:


Contacts (if you set them in iCloud via your iPhone)
iCal Calendar (if you set them in iCloud via your iPhone)
Email (if activated) - this will be an @icloud.com or @me.com address
Notes (if you set them in iCloud via your iPhone)
Reminders (To do stuff)
Find My iPhone (NEVER ever turn this off, except if you've sold your iPhone)
Documents saved in iCloud via Pages, Numbers and Keynote