Thanks, that helps me understand it better.
So when me and my friend leave the room because her dog is jumping and biting us it's negative reinforcement, because the dog is like "why are you leaving" but after a few times they associate you leaving with their jumping and biting so they stop. Is that right?
Glad to help
What I would do with your friend's dog is to 'distract' with a toy/treat (I wouldn't use noise distraction though) and CLICK/TREAT when the dog takes the treat/toy. Also ask the dog to sit or lie down if possible. This is a bit harder because with a dog leaping and biting it's painful on you and your friend.
And the dog may have 'zoned out' on you.
If necessary, dog leaping and biting a bit too much for close contact, throw the treat/toy and click when retrieved and again
throw the treat. That way the dog will get the message and you'll be 'safe' from unwanted attention. Then S - L - O - W - L - Y get closer.
If your friend's dog brings the toy/treat back to you, then ask for sit before click/treat.
Your friend's dog sounds like it's either (a) a youngster (b) loves to play and wants you both involved in the 'game' or (c) both of the above
Don't encourage the dog to 'play' with you either, stay very calm and quiet. Too much excitement and noise will make a lot of dogs worse, it get's them all excited too and they show it by leaping and biting, it's THEIR way of having FUN - but not yours
You're right about how the dog thinks when you leave the room, but the dog will still remain 'stressed' even if the dog does stop eventually. He/she will be more into 'shut down' mode than a good behavioral response. Still wondering 'why' but giving up jumping and biting and this is not what you're after achieving long term is it? You want your friend's dog to sit or lay down with you both HAPPY AND CONTENTED!
Example:
Visited an elderly lady yesterday. She has her granddaughter staying with her and the granddaughter's dog is pretty much untrained
Small dog but brother CAN IT NIP!!!!!!!!! Caught me a beauty on the leg, I didn't move fast enough to avoid the snapping jaws
This dog does this EVERY TIME you visit!!!!!!!!!! And what they do is SHUT THE DOG OUTSIDE!!!!!!!!! Hasn't stopped the unwanted (and painful) behavior AT ALL!
You still have to do some 'fancy foot work' to avoid a decent bite/nip and believe me this dog's nip is darned painful!
It broke the skin on my leg and I have a seriously decent bruise this morning!
So as you can see,
shutting the dog out, or removing yourselves, isn't the answer at all.
I have offered to help the lady with this problem, not sure she quite 'got it' so will wait until I see the granddaughter, because it only needs that dog to bite someone else, who's not so tolerant, and well....... results will not be very good
This dog is an older dog, so it's not playing, it's stressed about visitors and 'resource guarding' the home. But the same principal applies,
just removing the dog hasn't worked!
From bitter experience, and I still back-slide, only way to get a message across, when angry, is in quiet well modulated tones (with a bite). Postulate answer first, don't open mouth until you're sure you can speak without yelling, take several deep breaths and failing all else walk away with mouth firmly shut
Just the act of walking away does at times get your message across
! 
Adore them
Thank you for pointing that out. 
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