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While it might be considered like an extended trip and in fact corresponds to the maximum length of time allowed under visitors visa in the US or UK for example, a 180-day stay in the Schengen area falls under completely different rules, more akin to those for long-term residence. This means that everything you are reading about short stays (the Schengen rules, the 90/180 limit, the visa waiver agreement) does not apply here.
Until now that person had a national long-stay visa and she will need another one to come for another 180-day period. As far as the Schengen rules are concerned, Malta is completely free to issue two of those back-to-back and whether they had be willing to depends solely on Maltese law.
Interestingly, she could also come back immediately for up to 90 days under short-stay rules because time spent under a national long-stay visa does not count as far as the Schengen regulations are concerned. Those things are entirely separate.