what i did was, i made the online appointment 2 weeks before my intended day of submission.. then a day prior my appointment, i went to BDO and asked for a manager's check drawn... but BDO will only issue you with a manager's check if you have an existing account with them (which, thankfully I had).. not sure with BPI's policy on that though...
what your fiancee plans to do is ok... there is a bdo bank right accross vfs so she can actually pay there and bring the receipt to vfs.. if her appointment is in the afternoon, she can pay in the morning and submit her documents in the afternoon...
good luck!