Billy and Tara's struggles and hopes did not get a good result. The marrow of the two matches Jenna, it's just that Billy who has heart disease and Tara who enters a vulnerable age make Doctor Armando not allow them to donate to Jenna.
This certainly broke the hearts of the two. They cannot imagine the days without their only daughter. However, if Jenna came to find out that they were trying to donate marrow to her, the girl would also be very heartbroken.
In the end, they promised each other to keep it a secret so that Jenna didn't know anything about it. They even asked Blake to keep everything a secret. Until Jenna's condition improved and allowed her to go home.
Doctor Armando only added the dose and type of treatment of medicines that the girl had been taking. Which indirectly forced the cells in his body to regenerate and made her survive longer.
