Inspired by the comment of @AnthonyX to this answer: Could the Apollo's CM pilot (who would normally have stayed in orbit) have switched to the role of LM pilot/commander if one of the LM personal would have developed a medical emergency ("a really bad cold") enroute to the Moon that prohibited him from landing but permitted him manning the CM? Were they cross-trained?
No, they were not cross-trained and regardless the mission would never have proceeded in such a circumstance.
As already noted by others, it was the commander and not the lunar module pilot, who actually flew the lunar module during landing.
Training for this was supported by the Lunar Landing Training Vehicle. From Unconventional, Contrary, and Ugly: The Lunar Landing Research Vehicle (quoting MSC Director Robert R. Gilruth):
The present plans call for a minimum of 22 flights for each Commander and backup Commander assigned to lunar landing missions. This includes 11 flights for initial qualification and 11 additional full lunar simulation trajectories for proficiency. We do not intend to train any Lunar Module pilots in the LLTV...
Our current assessment of LLTV support for Apollo 13 shows adequate capability to provide proper training for the Commander, with marginal capability to support the backup Commander...
Unlike LM commanders, who trained in the LLRV/LLTV, LM pilots received training in the Lunar Landing Research Facility (LLRF) at Langley. Training in this enormous outdoor structure was considered sufficient since the likelihood was remote that an emergency might develop during a lunar mission that would require the LM pilot to take over from the commander (who actually flew the LM) during the final 500 feet of the lunar-landing trajectory. Due to the extensive maintenance requirements on the LLTV and the shortage of skilled crews for operations, training of backup commanders was a low priority.
So, in short: even the LM pilot was not really properly qualified to land the thing!