Did Sheridan and Delenn ever learn that Emperor Londo Mollari was controlled by the Drakh?

Question

Londo Mollari has a Drakh keeper attached to his body in season 5 of Babylon 5. The Drakh control him and get him to enact policies that further isolate the Centauri from other races.

His friends, including John Sheridan and Delenn, worry about his well-being. Did they, or other friends of Mollari, ever learn about the Drakh keeper?

Answer 1

Per the novel Out of the Darkness, they learned it in 2278 when David Sheridan II, possessed by a Keeper kept in an urn Mollari had given his parents, awoke and compelled him to journey to Centauri Prime. When John and Delenn travel there to rescue him, they are captured by Mollari. In the novel (from the Keeper link)

Garibaldi went to Babylon 5, where he was able to learn from Vir Cotto that the Drakh had been on Centauri Prime for quite some time, and that they were in control of the Emperor and Prime Minister.

Even if we discount the novel, War Without End Part 2 more or less implies they know about the Drakh by then, and older Delenn states that their son (David) is safe (meaning the Keeper was removed or at least discovered).

Answer 2

To add some context to Machavity's answer, these are some brief highlights from the Legions of Fire novels...

• Londo was fitted with a keeper mostly to keep him in line. The Drakh used more subtle means to put people with the vision they wanted in power in Centauri government, while Londo rubber-stamped the early work to get it moving. Where possible he was left out so that he could not interfere. So opportunity to detect Londo's keeper were pretty scarce.
• Vir knew that the Drakh were present on Centauri Prime long before most did, and knew that Londo was under duress. He is in fact largely responsible for the Drakh's downfall. However, with the Alliance races none too trusting of the Centauri, and his very limited resources, he forced himself to limit the spread of that knowledge so as not to give the Alliance another excuse to attack Centauri Prime.
• John and Delenn knew something was wrong... but not what, or who, and came to much the same conclusions Vir did. They couldn't intervene directly without Alliance resources, which could lead to too many disasters.
• Garibaldi became involved in Alliance monitoring of the Centauri at John and Delenn's request, and he brought on Lou Welch for some underhanded digging. The Drakh were well entrenched at that point however, and this only led to Lou's murder. In the first open act of his little resistance group, Vir assassinated those responsible to keep Garibaldi from dismantling the place.
• Timov renewed a relationship with Londo - they were still married after all - but when they got close enough for her to be a target, Londo arranged a frame-up and exile for her. She believed his new ministers were behind it.
• G'kar was a prisoner for a while, after saving Londo from an assassination that he was then blamed for. G'kar knew about the Drakh from Vir, but he still didn't know about the keeper until Londo showed him shortly before they both died.