I'm going to give my friend's blanket to an injured woman
What happened to make you want to give it away?
I don't have any. It was an accident I caused myself.
While you did the right thing by not just standing there and doing nothing, that item was still your friend's. It might have been important, so you should have taken it away when the ambulance arrived. It wasn't your item to begin with so your friend might get angry at you if you didn't get another one.
Fuck, my friend is going to be pissed at me. Should I tell him?
If you have no other options it is good in an emergency to use what is closest to you. You should tell then and offer to replace the item, I think they will be understanding if you tell the the reason why.