She has been sleeping with him many times

Well then more information is needed(like how long, last test, last whatever), she may be in the process, just waiting to turn +ve, which is not a special case as people don't turn +ve immediately, it is very likely she will test +ve in time if she is not on ARV.

Yeah, smano, thats another possibility, another one is that it is a cock and bull story made up to attract attention to the column.

It is possible!

There is a terminology 'Transmission probability per exposure event' which can explain the lack of infection of this chick despite many exposures which can be as a result of the following:
i) The girl may have a genetic advantage and lack full expression of CCR5 co-receptor either as homozygote or heterozygote. In wazungus its delete 32bp in Africa delete 24bp.
ii) the guy is on ARVs and has not developed resistance so the viral load is very low....the higher the viral load the higher the chances of transmission.
iii) The site of landing..... if the guy lands on the real airport (vagina) chances are 1 in 200 to 1 in 2000 exposure events..depending on viral load and the type of HIV that the guy has.. Remember all HIV viruses are differences and infection or transmission events are different.
If the guy lands on the intestinal tract (plugs+sockets) the transmission is higher 1 in 20 to 1 in 300.
Kindly note that these are probabilities.

@ McReggae. Zuma went for one term (Hit and run). The chic mentioned in the link said she has a a kafry so many times!

@ Smano. Does it mean that if a guy does no offload the swimmers, he'll be safe coz the epithelial cells are not exposed?

Following the analogy of suckers willing to attach,it will mean that women need to fly just once and GBAAM..they have it.

T cells... that's interesting. Can you expound on this?

@ Much Know. The uninfected lady on ARV? ARV is for the infected(if I'm not wrong).. thought the pre-exposure drugs are called PRP or something. How safe are they? Anyone?

@ BGL. Interesting.... any link?