A:
I'm assuming the voting was done by ballot.
Votes -- legal or illegal -- are only cast by members eligible to vote, so the ballot cast by the non-eligible-to-vote member is simply thrown away if it can be identified as coming from him.
If his ballot can't be identified, then the election is null and void if his "vote," whatever it was, could effect the outcome; otherwise, the election stands.
(See RONR p. 410.)
FYI, "illegal votes" are votes cast by members eligible to vote, but are faulty in some way, such as (a) a ballot has more votes than are allowed, or (b) a ballot has a vote for a person not eligible for election, or (c) the vote is unintelligible.
Merely mispelling a name does not make a vote illegal unless it causes an ambiguity; e.g., if Smith and Smyth are candidates and the vote is for Smitty.
Illegal votes are counted in the number of votes cast.
So, for example, if the votes cast were:
Clinton, 10
Dole, 9
Mickey Mouse (Illegal: fictional), 1
there would be no election since no candidate received a majority (more than half) of the votes cast (20).
The election would have to be done again.