Someone else's photos and search results appearing in my phone after iCloud backup

After doing a quick search this looks like an ongoing problem and something isolated to iCloud but mine is even more bizarre.

My friend lost her MBP recently and was desperate to back up her iphone 5. I signed out of my accounts on my MBP and signed into hers (including her iCloud), deleted the old backups (as her storage was inexplicably full despite the iPhone backups taking up far less than 1gb). It backed up via her phone and I signed back out of her accounts.

A couple of days later and my iTunes keep automatically signing me out of my session annoyingly. My phone won't sync my photos to my iPhoto folder any more saying doesn't recognise three photos. My iChat keeps signing me into her phone automatically and a google account I didn't even know I had! Plus now her numbers have appeared on my phone, her photos appear in my iPhoto stream and my photos are popping up on her phone??!! My MBP is yosemite as I refuse to update losing vital programs and simple things like disk utility not being able to raid a drive or erase sxs cards (essential for my job). My phone is an iPhone 7.

None of this makes any sense! How best to resolve other than restoring the OSX? And will that even cure it?? Anyone else having this odd conflicting thing? Very very worrying that we're getting each other's personal browsing history results, photos and phone numbers...