There's a PHP setting that specifies how much memory a script is allowed to use. When this limit is exceeded, you get that error. In this case, the limit is 32MB, and the script tried to use 35 additional bytes.
It's hard to say why that would only be happening for him, but increasing the memory limit for SMF to 33MB should fix the issue.
Add this code to the beginning of index.php right after the opening PHP tag (on its own line of course):