Valeria should already know because she's smarter than Reed.
I was thinking that the future Franklin and Valeria should have know, but their master plan was to save their parents from getting killed a few months earlier, so the timeline reset and they became just as ignorant of the future as everyone else... Although Grimm is supposed to live for hundreds of years more if we believe that story from a year ago... So if Ben was/willbe dying of whatever Reed is, then Franklin and Valeria in some other Future already got on top of that malady before they time travelled back to a year ago.
Reed is in possession of the yellow reality controlling infinity gem.
He can just wish himself well, since both his body and whatever is wrong with it are aspects of reality.
This is so pretty...
Although why is beast, who was also dying of an incurable disease, afraid of Professor X from the 1960s if he's got the Mind Gem in his back pocket? Professor X shouldn't be able to do jack to him since he should be able to control most of the universe if his imagination is up to the task.