https://github.com/syohex/emacs-helm-robe.git
git clone 'git://github.com/syohex/emacs-helm-robe.git'
helm-robe.el provides function for setting robe-completing-read-func.

Set helm-robe-completing-read to robe-completing-read-func as below.
(custom-set-variables
'(robe-completing-read-func 'helm-robe-completing-read))