There is an idiom in Persian that literally says: "(to be ignored right after) somebody's donkey has crossed or passed (over) the bridge"
We use it in situations where someone (now feels relieved and) ignores or treats us much differently after:
We have helped them to overcome or reach a solution to a problem, but as soon as they feel relieved, they start treating us as if we are now strangers or even annoying to them! (i.e. They totally forget the help they received.)
We have provided them with something they needed (like a piece of information or news, a confirmation, an acceptance or agreement, money, etc), but since their need has been met, they don't think twice about ignoring us.
As you know, donkeys are stubborn and when they perceive danger, you can't force them to do anything. So I think the origin of this idiom might have been like this: someone's donkey refrained from crossing the bridge, so they asked a friend to help them get the donkey over the bridge, but after being on the other side, they forgot about their friend or treated them ungratefully.
- Your friend calls and desperately asks you to lend a large sum of money otherwise they will go to jail. They promise to pay back the sum in two months. You lend the money and they say you are their best friend.
After three months they still haven't paid you back. You need the money, so you call and ask them when they would repay it. They promise the following month! A month goes by and still no sign. So you call them again on the 6th month. They get angry and scold you about why you have called them over that trivial (!!) sum of money and ask you not to waste their precious time with these annoying phone calls since they would definitely pay you back as soon as possible! At this point you say:
"Okay, you are talking like this because your donkey has already passed over the bridge and you feel safe and relieved now, but are you sure it was your last problem??! I will never help you anymore even if you are about to be executed!"
- Joe and Jane have been dating for five years, during all those years Joe's behavior has been exemplary; he really wanted to marry her (so he was just acting according to Jane's wishes and requests). After Jane accepted his proposal of marriage and they got married, Joe started to show his other side and gradually ignored Jane's requests. So Jane told him sarcastically:
"Oh, yeah! now that you've got my "yes", you are ignoring me and the reason is clear: your donkey has just crossed over the bridge!"
Is there any equivalent idiom, expression or proverb in English to criticize such people?