HIV is a virus which can be transmitted through the exchange of bodily fluids except saliva. There are several theories why it can't be passed through saliva maybe it's the enzymes or glycoproteins in it which kill the virus, this means it can't be passed through kissing.
The most common methods are through exchange of blood ie through cuts, razors, shared needles or blood products.
The other is sexually - through semen and vaginal fluid. Unprotected intercourse carries a high risk - higher for women of men receiving anal sex. Circumscised men carry a lower risk of contracting, although it still exists.
Oral sex carries a risk, lower than intercouse, of transmission, bleeding gums and ulcers will increase this risk.
A baby born to an infected mother will also without preventative measures become infected. Breastfeeding off an infected women will also cause transmission. HIV Tutorial
So yes your wife can get it through another woman from oral sex but it is less likely than through other methods. Not many studies have been done succesfully on the transmission rates of oral sex as it is hard to isolate a group who only have oral sex.Oral Sex: What's the Real Risk
EDIT this is a really good site to have a look about.
I hope this clears things up for you, if I have missed any other questions you would like answering just post back and I'll expand on the subject.