If the company didn't actively pay her to develop such a DB then it's her IP.
The biggest caveat for your wife though is supporting the application (and doing her normal data capturing job) and making changes to it at the request of the company (which they'll obviously have to pay for). If she has limited exposure in anything "dev" related, the task of maintaining/supporting and developing the database to their needs/requirements to keep them happy will become a fulltime job in itself. So be careful when you start charging a monthly fee for it or selling to other people.
What I would do is to sell the IP entirely to the company. This way removing any real responsibility of maintaining/further developing the database from me while I do my normal data capturing job. If and when they do require support or development/extra features, charge seperately for it based on the time I spend on it after working hours, or come to an agreement that, with a payrise, I will maintain/support this application and that any new development work would be done during office hours IF her appointed work (capturing data) will not be affected. If it's affected and they expect her to perform with her primary role still, charge for working on it after hours.
10 hours work is roughly R4000 (what I usually charge, yea, not rolling in it, but it keeps the idiots at bay with the price and normally gives me the guys who's serious), but I'd up that figure to R10k including the IP.
It won't cost them that much to contract someone in India to replicate what she's done. So be careful.