Either have the client request proof from the previous accountant  or  file the extension yourself for peace of mind.    If it were me, I'd file an extension.     Do  you really want to take the chance it wasn't done and deal with IRS notices later on.

If you try to e-file an extension and it goes through, no prior extension was filed.

If you try to e-file an extension and it is rejected with a message that a tax return or extension has already been filed, then you probably don't need to worry about anything.

Or sent Form 7004 by mail, postmarked today.

First time filing an S Corp return and trying to request an extension...getting  an error code F1120s-239-01  Basically telling me the filers EIN and name control or address change needs to be corrected.......goes on and on.
Apparently its common the first time a return is electronically filed for the S-Corp

Need Help,  I've tried multiple attempts to correct it. 

Has the client received their S-Election acceptance letter?   If not you cannot efile SCorp extension, it will need to be mailed