Is it permissible to relay words spoken about a colleague at work, which might cause him material or moral harm, in order to enable him to defend himself, or is that considered backbiting?

The questioner should have defended his brother's honor in his absence and clarified his innocence from what was attributed to him, if he was indeed innocent. This is based on the established encouragement to defend a Muslim's honor and the severe warning against abandoning him. If the speech leads to harm, such as dismissal from work, it is permissible to inform the colleague about what was said about him so he can be wary of the speaker and refute the accusation. This falls under the category of sincere advice (naseeha), not forbidden tale-bearing (namimah). However, if the harm can be averted without identifying the speaker, that would be preferable.