Thank, eponine. And, Addy, what you're doing by accompanying your friend is a mitzvah.
I once read an article about circles of pain in the families and friends of chronic-disease patients: There is the patient himself or herself, then the spouses/parents/children, then friends/relatives, then acquaintances/colleagues/neighbors. All can feel sorrow or pain or anguish, but those of us on each successive outer ring should always treat those on inner rings with greater respect.
So to be there for your friend who lost her husband is an act of great, great friendship. You will have your own grief, but in helping her cope with hers, you will be doing something very powerful, even if you never discuss it.