It sounds like your friend's current iMac is a 21.5", Mid 2011 model which, according to Mactracker, can run Apple's latest OS, 10.12, Sierra, and I've read that all Macs that can run Sierra will be able to run Apple's next OS, 10.13, High Sierra.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire