The therapy treatments are for numerous diseases and genetic conditions, not just life expansion. Besides imagine that it may not be that hidden and Revy could find out some way. After all Samara did tell Shepard about it in canon.
@tri2 Is it fair to assume that if Revy does the treatments for the Asari that numerous Asari would want to tell Revy about the disease in the hopes of finding a cure? What considering not just the numerous families with members afflicted with it but the Asari themselves wanting to be rid of the disease for obvious reasons. Because at this point seems like Revy has a reputation for being able to do near anything if motivated and she has already shown to be able to work miracles with genetic science for numerous races.