Jason,
What happened between you and the young woman, was not a grief thing. Well, perhaps grief did cloud your judgment, then again perhaps not.
Your son's girl friend has forgotten that she is a quest in your home. Were it not for her relationship with your son, she would not be there. When a person says they are moving out, they are playing on your love for them. This is often done by children, your children, not a guest, and it is self serving.
The girl friend is you son's problem, not really yours. It is he who needs to have a conversation with the young woman's attitude, not you. Also, keep in mind, as the young woman should do as well, that a girl friend is not a commitment by the son. For a young man, girl fiends come and go, and his current relationship is not necessarily going to be a lasting one.
The only issue I see you having is what will occur when your son goes off to college. Is the girl friend following or staying with you? You need to give this a lot of thought, and again, your son needs to be involved.
I know this is all hard for you, and would be a lot easier if your wife were still around, but with some introspection, I believe you will find a solution. After all, you knew your wife well, and deep down, you know what she would have had to say.
Jason, you are a good man and do not need to deal with an 18 year old and hamsters.
You have a son who needs help getting off to college. It is a time, as you have said, your wife would have enjoyed. It is a time that now you have to enjoy for you both. Not an easy thing to do, but I believe you will find a way.