I had to login to my Paypal from my friends phone and he's logged out since but he still continues to receive notifications when I make payments. I've since logged out of all devices and he's still getting notifications on his iPhone for whenever I buy things.
I am sorry to know that your friends receives the notifications of your account. Please ask your friend to clear the cache memory of the application and kindly ensure Google has not synchronized your account (if the mobile is android).
Please ask them to check his phone > settings > account > remove your account if added.
