The company is not required to inform the immigration. This is different from the case of domestic helpers, where employers are bound to inform the immigration dept of termination of their employment.
In the case of your girlfriend, she is allowed to stay here till the end of the visa expiry and can leave and re-enter Hong Kong freely till that date. After that she will be able to enter as a visitor only.
If she were currently under the employment of the company, at the time of expiry they would send in a letter requesting an extension of stay thus allowing her a renewed visa.
The company will most likely not send any letter to immigration but even if they do, your girlfriend by law can stay till the expiry of her visa shown on the passport and the immigration will honour that.
If she obtains employment elsewhere, she should get them to send a supporting sponsorship letter and renew her visa at any given time before the expiry of her present visa. It is easier to do it when she is in Hong Kong itself rather than from overseas as that takes a lot of time via the Chinese Embassy etc.
Alternatively you could marry her and she could get a dependant visa on her passport.