From another piece about same study (and I like the way this was phrased):
Negative validation, the study explains, “communicate[s] that the feelings, actions, or responses of the recipient are normal and appropriate to the situation” and “express[es] appreciation for the recipient’s predicament or for the difficulty of the situation.”
Negative validation won. People with low self esteem felt more affirmed and supported by just being heard and validated. They did not want to look on the bright side. So, practice saying, "It's normal and okay for you to feel sad. That's really hard."
If you just can't help yourself from offering the optimistic view, better to leave your depressed friend alone to wallow in their misery. This approach also has the side benefit for you. The researchers found that people who try to cheer a friend up and fail end up feeling worse about themselves, too.
It still seems off to me, though, to say this is a low self esteem issue.