JD, I think you are referring to something which Pope Benedict wrote in his book Light of the World. Because it was widely misinterpreted, the Congregation for the Doctrine of Faith issued a clarification. The pope was not endorsing the concept of "harm reduction. Providing condoms to a prostitute is cooperation with a moral evil, namely the sin of fornication. We should focus our efforts at helping prostitutes escape from that lifestyle.
A prostitute who is already infected with HIV may use condoms to avoid harming others. However, this should merely be the first step to quitting prostitution. It can not be a means of avoiding HIV to continue working as a prostitute.
I am going to provide a lengthy quote from the CDF article, but there is more at the link:
*On the pages in question, the Holy Father refers to the completely different case of prostitution, a type of behaviour which Christian morality has always considered gravely immoral (cf. Vatican II, Pastoral Constitution Gaudium et spes, n. 27; Catechism of the Catholic Church, n. 2355). The response of the entire Christian tradition – and indeed not only of the Christian tradition – to the practice of prostitution can be summed up in the words of St. Paul: “Flee from fornication” (1 Cor 6:18). The practice of prostitution should be shunned, and it is the duty of the agencies of the Church, of civil society and of the State to do all they can to liberate those involved from this practice.
In this regard, it must be noted that the situation created by the spread of AIDS in many areas of the world has made the problem of prostitution even more serious. Those who know themselves to be infected with HIV and who therefore run the risk of infecting others, apart from committing a sin against the sixth commandment are also committing a sin against the fifth commandment – because they are consciously putting the lives of others at risk through behaviour which has repercussions on public health. In this situation, the Holy Father clearly affirms that the provision of condoms does not constitute “the real or moral solution” to the problem of AIDS and also that “the sheer fixation on the condom implies a banalization of sexuality” in that it refuses to address the mistaken human behaviour which is the root cause of the spread of the virus. In this context, however, it cannot be denied that anyone who uses a condom in order to diminish the risk posed to another person is intending to reduce the evil connected with his or her immoral activity. In this sense the Holy Father points out that the use of a condom “with the intention of reducing the risk of infection, can be a first step in a movement towards a different way, a more human way, of living sexuality.” This affirmation is clearly compatible with the Holy Father’s previous statement that this is “not really the way to deal with the evil of HIV infection.”
Some commentators have interpreted the words of Benedict XVI according to the so-called theory of the “lesser evil”. This theory is, however, susceptible to proportionalistic misinterpretation (cf. John Paul II, Encyclical Letter Veritatis splendor, n. 75-77). An action which is objectively evil, even if a lesser evil, can never be licitly willed. The Holy Father did not say – as some people have claimed – that prostitution with the use of a condom can be chosen as a lesser evil. The Church teaches that prostitution is immoral and should be shunned. However, those involved in prostitution who are HIV positive and who seek to diminish the risk of contagion by the use of a condom may be taking the first step in respecting the life of another – even if the evil of prostitution remains in all its gravity. This understanding is in full conformity with the moral theological tradition of the Church.*