In general they can finish their post if they're in the middle of it (at least that happened to me as an Admin and I was able to finish my post - but it could have been due to me being an Admin)
I'm not sure as I've not looked to see what command is issued to MySQL so depending on what is issued would be how the user is affected and maybe the version of MySQL.
Then again he could be setting some setting via PHP to temporarily enter a value that would stop further open/write commands.
