Hi Yunfeiyang
Welcome to GMATClub.
It is quite difficult to rank PhD programs. It is also difficult to answer the very general query you have put up, however, I will try.
Marketing PhD programs look for candidates in roughly two streams - quantitative and behavioral. I suspect the latter is what suits your gf. Good Programs in behavioral marketing are Harvard, Kellog, Columbia, Wharton, NYU, Cornell, Insead. Flavour at MIT, Texas Austin, Carnegie Melon etc tends towards quantitative.
I am not sure how to answer placement question in general, but PhD placements at most of the top 20 univs are good. US univs hire their own (ie PhD's from other top univs in US only), while European univs are open.
Tough

I am not sure how to rank. Probably the toughest PhD program is that of Rochester (Simon School) where routinely 50% of entrants drop out. However, candidates from all backgrounds enter PhD programs at various universities and survive, so it should not be a problem with your gf ! Best of Luck.