To be honest, this is probably the least harmful lie revealed in manga.
Of course it doesn't make it ok since I'm 100% sure he would have accepted her even if she told the truth at the very beginning.
.....in another light, she kind of stole her friend's lover. But since this was intended to be a "package deal" (thank you
@Darketower for that analogy), again I really don't see anything wrong with it.
Poor dark elf girl though.....your fiancée is happily banging another girl everyday and plans on adding another (two) to that.