Do I incur sin or legal liability for having directed my friend to an intermediary I had dealt with previously, and he subsequently experienced an issue with this intermediary of which I was unaware, and must I compensate my friend for the loss he incurred?

What you mentioned does not necessitate a guarantee. Merely pointing to an intermediary and vouching for them based on their past dealings does not involve deception or fraud that would warrant blame or responsibility. The fundamental principle is the innocence of one's liability until proven otherwise.