When would it be appropriate to disagree (in a publication) with a paper you have proofread?
The fact that you proofread the paper does not change anything: once the paper is published it is available to anyone who wants to criticize it.

The most appropriate course of action however is to try and correct the error in collaboration with your colleague: either by publishing an erratum or a new paper. Or maybe you will decide (after discussion with said colleague) not to correct the error, eg. because it is not important enough. Your colleague will probably not appreciate it if you publish a paper about him being wrong without discussing with him first: doing so is one sure way to destroy any friendship or future collaboration.

Secondly, simply pointing out that another paper is wrong is usually not enough to get a publication.