A couple clarifications: it was the cancer that had the unique mutation, Henrietta was genetically healthy. Secondly, it wasn't an experiment, they took a biopsy: an important distinction because black people were literally experimented on[1] from 1932 to 1972. Finally, the Wikipedia page downplays the racist element in the biopsy, claiming that "At that time, permission was neither required nor customarily sought."[2]
Even if it was standard practice, it is not fair that pharma companies are making billions off of her cells when her descendants are struggling to get by
Market valuation is determined by supply and demand. HeLa cells are substitutable goods in that if Henrietta asked for money for the cells, it would have been easy to find some other donor.
Also it's overly simplistic to say pharma companies are making "billions" off her cells. HeLa is a part of the workflow but far far far from the only part. Also value is in the work of scientists understanding the cell biology using her cells as a model, not some inherent value in the particular genome of HeLa.