she can - but it depends how she wants to be perceived.
i never wear strappy tops but others do

a qatari lady thanked me the other day for not wearing shorts on the corniche
she was very educated - and she appreciated me showing respect to her country. we had a lovely chat.

no-one will say anything if your fiancee does - but it is appreciated tacitly if you don't

anyway if yr fiancee is coming from SA tell her to bring heaps of ostrich sausages from pick n pay.... yum yum yum