We were having problems with the basement floor drain not draining, so a friend (who worked for Roto Rooter) came over and unplugged the "overflow plug" and said that would fix it.

Another friend at a later date said that was dangerous because of methane gas coming up - can you tell me who is right?

Another friend at a later date said that was dangerous because of methane gas coming up - can you tell me who is right?
Your friend's correct. Removing the clean out plug opens up a direct path from the sewer to your house allowing sewer gas to enter. The trap of the floor drain's clogged. Replace the plug and snake the floor drain trap. Ya know, there are two reasons Roto-Rooter and big box plumbing department salesmen are'nt out in the field doing plumbing. Knowledge and experience .