Purely from a legal academic viewpoint the '4 male witnesses requirement' implies they are prosecuting for zina adultery (the only use of it in the quaran is where someone has accused a woman of adultery and the accuser is forced to provide 4 eye witnesses or else be charged with slander). If adultery is proved by these witnesses the sentence is death for both.
The treatment of rape as zina stems from 1979 when much of the shariah laws were given legal statute by the dictatorship in Pakistan as part of the Hudood Ordinance. It is now cited as the main reason for 80% of women in prisons in islamic countries being held.
There are several islamic scholars in Dubai currently arguing that rape should be treated as hirabah terrorism/warfare citing a hadith where a woman is raped with no witnesses (she is free to go after identifying him, he eventually admits it and is stoned to death) with the lawyers arguing that medical evidence can now 'admit guilt' for him. If hirabah is proved the sentence is death for the rapist.
It seems likely that treatment of rape as a subset of adultery is being used to invoke the 4 witness requirement (which of course can never be satisfied - in no believable circumstance do 4 people watch a rape, do nothing to stop it, then willingly stand up and act as witnesses) and by doing so avoid having to enforce the death penalty for the rapist (who here is also an ex pat).
They (Dubai Authorities) are between a rock and a hard place - either they
hold both in custody and then persuade her to drop the charges when 4 witnesses cannot be found or they put a westener to death which would damage their ablity to attract the foreigners that are their life blood.