Native Mandarin speakers who wish to teach Mandarin, but whose teaching qualifications are for subjects other than Mandarin, are generally accepted as qualified for the job (particularly if they have years of relevant experience), however they would most likely not realise the same level of pay as someone with a Mandarin teaching qualification.
Many mainlanders in the same boat are encouraged to take a course in Teaching Chinese as a Foreign Language at one of Hong Kong's tertiary education establishments. A typical course could last two and a half years if undertaken on a part-time basis.
This would not only boost your wife's salary but also her attractiveness to prospective employers. She should be able to teach and study at the same time if she does things part-time (and if her visa allows).
It is also worth noting that your wife will need Hong Kong qualified teacher status in order to teach here. She can apply for this herself, or a school can apply for it on her behalf. The process of getting this status, even for a teacher with a recognised overseas qualification and years of experience, is not necessarily a short one, however as long as certain conditions are met, your wife would be able to carry on teaching whilst she waits for her Hong Kong teacher status to be approved.