When someone asks you for your opinion of someone they are looking for a shortcut, in most instances, you can't provide.
Unless you intend to influence their view, it's best they make their own mind up. They will have a different relationship with the person they want to know about.
If the person in question is a toxic person, they will eventually find out for themselves. In most circumstances, they need to see it for themselves to believe it.
The added benefit of allowing nature to take its course is avoiding gossip or being the scapegoat when (and if) things go south.