Personally I think its PEBKAC but this person never has had any issues before.
I thought that the keyboard layout may have been messed up, but everything checks out.
To top it off they asked me to replace the laptop as they wanted to upgrade it regardless. 
And the new laptop is doing the same thing (for the user)
I've looked at it and can't see anything wrong.
Still don't know what apps the user experienced this issue with, but all I transferred over from the old computer was some documents and photos and reinstalled office 365.
Seems pretty coincidental to me that 2 separate computers would do the same thing.
Apparently pressing a full stop gives the letter y and rebooting fixed it.
Interested to hear if anyone else has heard of something similar.
Either way, I can't fix something that I've been so far unable to replicate!
I thought that the keyboard layout may have been messed up, but everything checks out.
To top it off they asked me to replace the laptop as they wanted to upgrade it regardless. 
And the new laptop is doing the same thing (for the user)
I've looked at it and can't see anything wrong.
Still don't know what apps the user experienced this issue with, but all I transferred over from the old computer was some documents and photos and reinstalled office 365.
Seems pretty coincidental to me that 2 separate computers would do the same thing.
Apparently pressing a full stop gives the letter y and rebooting fixed it.
Interested to hear if anyone else has heard of something similar.
Either way, I can't fix something that I've been so far unable to replicate!