All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no harm for this girl who is tested HIV negative to marry someone who is tested HIV positive as long as she is aware of the matter and is willing to marry him.
What is Islamically forbidden is for him to hide that from her; in which case he would be deceiving her.
Abu Hurayrah narrated that the Prophet said: “He who cheats us is not one of us.” [Muslim]
It is known that there are many Islamic objectives in marriage, so it is not limited to sexual intercourse and the satisfaction of sexual desire, but there is financial support, care, and being friendly and amiable to each other, and there are other noble objectives.
Both spouses should strive to prevent harm, by consulting specialist doctors in the field in order to obtain advice from them in this field, such as using condoms, antivirals, etc.
The wife should help her husband in search for treatment, and it has become possible and available at this time and age more than ever before.
For more benefit on disease that gives the spouse the choice to nullify the marriage contract, (in case they were not informed about it), please refer to Fataawa 365911, 384409, 128182, 134855, 84591, 326140, 263554 and 195891.
Allah knows best.