aww I'm so sorry! My heart is breaking for you!
I wish I could tell you that dogs don't have the feeling of sadness, but that just isnt' true.
she probably will miss you and will feel sad. but only for a little while. eventually, she'll be attatched to your son and his girlfriend. I think the hardest part for them is that they don't understand why they're being taken away....they don't know what's going on. but we do.
when we brought Maddie home last week, the look in her eyes was so heart breaking. it was litteraly like, "where are you taking me?" but now, just a few days later, she's happy as a clam!
so, the point of this rambling, is that I do think their hearts can be broken, but I think they're also very adaptable and she'll get used to the change and grow to love your son and his gf just as much as she loves you!