Bealzebub, legally speaking you may well be 100% correct. The reallity is that no company is going to spend the time, effort and money to make a legal case for the amount involved here.
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If it's a return ticket then it's correct to point out that breaking the outbound sector may automatically cancel the remainder of the itenarary. If on the other hand he is doing this on the return sector, there will be no problem. Done it quite a few times myself (with another airline) and they're still quite happy to take my money and issue me tickets.