Try changing the call in spformproc.inc.php to $modx->webAlert(etc. ...). The webAlert() function is defined in the document parser so it’s part of the $modx class.
BTW, the fact that it’s trying to call webAlert there means that the mail function has failed. Usually, that means that the host has the mail() function disabled. If that’s the case, you can change the SPForm configuration to use SMTP.