She would have known (spoilers)

At the end of this movie, I was saying, "She would have known!" to my screen. Because as soon as she found out that Dawson had died, on the very night that the hospital found a heart for her son ... She would have known whose heart they were using. What's with that phone call a year later from her son to tell her who it was? And she acts shocked. And the son is so reasonable. Wouldn't most 18-year-olds be concerned that the heart now in their body had already had to beat for 39 years? Didn't seem to bother him in the least. Weird ending. Booooo.