Board Thread:The Flash discussions/@comment-9346948-20171101005520/@comment-24152851-20171101234930

SeanWheeler wrote:

Then why didn't Barry react when Wells mentioned Dibny? And if he was truly dead and Barry knew him, why didn't Barry say anything about Dibny being dead in this episode? That would be because Dibny's history was retroactively added in, so back in Power Outage the writers didn't know he and Barry knew each other.

In-universe.
 * For Power Outage. Either Barry already knew about Dibny's death so it wasn't a surprise to him, or the circumstances weren't convenient for Barry to display surprise.
 * For Elongated Journey: either Barry forgot (since he didn't keep in touch with Dibny for 5 years), or he wasn't ready to bring up Dibny's death at that time and it may get mentioned later.