my friends dog is to thin

I'd call animal control or take him to a vet.  You don't even need to tell her about it... just make sure he is healthy in every way aside from the food and keep feeding him yourself.  She either doesn't stop by often enough to see the dog's shape or doesn't care about him at all.  A dog does not become 40 lbs. underweight quickly.  This had to have been going on for a long time. 
 
Also, if you have the means, ask your friend if you can take over care of the dog and make him yours.  He probably needs human contact and love aside from food alone.

I would tell her you want the dog, and keep it.  Take it to the vet and then give it the love and care it needs and deserves.  To be honest, I dont understand why you cant mention something that from what you said is blatently obvious to your best friend.  I know you dont want to hurt feelings... but its like the elephant in the room, no?  And the poor dog is wishing you would do something for him.  Glad to hear someone is around to help him, and good luck!  BTW Welcome!

If I understand right the dog is now on your property and your friend comes there to feed the dog? If so regardless of who's dog it is I would think you are ultimately responsible seeing that it is being housed on your property. Even if legally you are not why would a dog be left behind a barn without human contact? If I misread something you posted excuse me. I myself would NEVER allow someone to leave there dog tied outside my property for any length of time especially without being properly housed and care for.

I feed my son's gf's dogs cuz she has no money, I would talk to her and say, "You know I saw puppy the other day is everything all right he looks thin to me?" She may say yeah I know I can't afford to feed him enough yada yada, and then ask her when was the last time he was at the vet? Again, I cannot afford it yada yada.
 
Then you just say you know I am sure it is hard, I would love to help puppers out do you mind if I just take him and get him food and vet care, I need another dog anyway to keep my youngest dog busy.....yada yada yada..........she maybe incredibly relieved.   If this does not work you HAVE to save the animal and call AC.
 
Keep us updated.
 
Julie

I don't think that sneaking it food would really help anyone.  I would talk to your friend about giving the dog a home with you.  She obviously has a lot on her plate right now & I'm sure she'd be relieved to have one less thing to deal with.  Especially if she's financially stretched you'd be doing her a favor & making a puppy happy.

I second everything Annie said.  Your friend may be relieved to have you offer to take care of her dog - one less thing for her to deal with right now.

I do think you took it the wrong way. I am not his provider, if i was ask to be fine, often My friends and family use my 140 acre farm to house thier dogs in till they are settled, they all have great housing, it is my houses. They still take care of thier dogs, I think that she is not feeding him as much as he needs for a dog his size. he does get human contact since my home is always full of lots of  people. I think by only giving a little here and there is not enough, for his size, and now it is starting to show. I love this dog he is a wonderful personality and has been threw alot of stress since the chanage, of his daddy being gone. I can not bring him because my female is tempermental about the inside of the house. and all my other inside kennels are full. I am not sure what kind of dogs you have but these are pits and since he is a full grown male, I can not leave three not fixed males togather. I can only have him every once in a while, when the others are out. I also have three kids so bringing him in is hard. I hate you got a bad vibe from it I want to help him and her, because i know they love him and are only in a tight spot and need some help. I am simply up set she did not tell me before now and let me help.

Momofthee,
You will find and do what is right in your heart to do. Personally, I would assume the responsiblity of taking care of this dog if your friend is unable too with all she has on her plate.
I also agree with everything Annie said too.
Good luck. It is never an easy situation all the way around.

Didn'y mean to upset you. I was just saying what I felt. G'Luck with this and whoever is responsible I just hope that someone takes responsibilty so that dog does not have to suffer.
FYI- We have a German Shepherd and English Pointer