After looking into this a little more it looks like the Ruby error is not a "parse error", though it probably should be? This might no longer be Ripper specific. Ruby just seems to give up on the above code no matter where it is placed in source:

$ ruby -e 'puts "one"; :~$ ; puts "two"'
-e:1: `$' without identifiers is not allowed as a global variable name

I would expect that if there is no syntax error that Ruby should print "one\ntwo".

You can handle the case by using compile_error instead of on_parse_error as below:

$ ruby -rripper -e 'class X < Ripper; def compile_error(m) puts "ERROR #{m}" end end; X.parse ":~$"'
ERROR `$' without identifiers is not allowed as a global variable name

IMO, ripper expects that all syntax errors are "compile errors", not "parse errors".
But current implementation of the ruby parser is not so.
Then, we must define both on_parse_error and compile_error to handle syntax errors at this time.

Ripper is just a thin wrapper over ruby parser, so the original parser differentiate them, ripper just inherits difference directly.
Extra work such as unifying two errors should be done by upper layer.