The onus is on her to prove to the satisfaction of Immigration that she is continuing to "ordinarily reside" in Hong Kong. It's at the discretion of the officer processing the application. Note that if it is rejected then by definition her ordinary residence is deemed to have been interrupted, and the PR clock resets to zero. So if this is important to you that I would advise not taking the chance - and do actually do what the law says you must (i.e. reside here properly), rather than trying to find ways to stretch things to the limit. In your case that would probably involve her living here, and you doing a M-F commute to Singapore (I think quite a lot of people do this).