Wikia prevents users from creating accounts with usernames that are already taken. I believe the reason for this should be obvious. In addition, Wikia also prevents users from creating accounts using usernames that are "too similar" to usernames that are already taken. I don't know the exact algorthm they use but the reason is that usernames that differ by only 1 or 2 characters could be easily mistaken for each other. The message your friend got regarding "Alexander46" is pretty clear; someone is already using that name. Regarding "Alexandr46", I would guess (but don't know for sure) that your friend is triggering the "too similar" condition.
Haya! It is a violation of ToU, link at the bottom of every page, to impersonate another user. Often this is done with usernames which are, as already stated, too similar. Not saying that is the intention of your friend, but there it is.
I too do not know the algorithm which blocks out one but allow some others, but it is there.