I'm so sorry to hear that.
You know, I don't think there are defined roles when it comes to funerals.
When my Sister's FIL passed away (though quite unexpectedly) she ended up doing much of what needed to be done. Her DH is not an only child, but no-one else seemed to be able to manage. She did all the funeral ringing around, arranged all the speakers, the wake etc. Her MIL was very grateful to her.
See if you can offer help. Whether it be physical support (assitance with driving, arrangements, heck even housework or a meal or three) or just letting her know you're there.
I'm certain she'll appreciate it either way.