You should be getting a "compilation" error in your Output tab whenever you try to update a snippet that contains something that has an error. GameMaker does not support coloring in the output log, so I don't believe I could make it substantially more obvious.
This only happens if you are trying to live-update a snippet with such a construct - it would not error if it's just "somewhere".
The other person's issue for now remains a mystery - they are yet to reply (on discussions or via some other contact method) nor have requested a refund.