Idiom or expression for criticizing people who ignore you right after getting what they wanted/ were after

The most common idiom I can think of is to say Now that he's got what he wanted out of me... followed by something like ...I shall not be able to rely on him any more.

Variations of this might include Now that I've got him out of trouble...; now that he doesn't need me any more... etc.

But sadly I can't think of any English saying which is equivalent to the donkey passing over the bridge.

A friend in need is a friend indeed

This phrase is interesting because there are various interpretations of its meaning.

There is some debate about the meaning of this expression. Firstly, is it 'a friend in need is a friend indeed' or 'a friend in need is a friend in deed'? Secondly, is it 'a friend (when you are) in need' or 'a friend (who is) in need'? If the former, then the phrase means: 'someone who helps you when you are in need is a true friend'. If the latter, it is 'someone who needs your help becomes especially friendly in order to obtain it' (emphasis is mine.)

The Phrase Finder

fly by night

(idiomatic, derogatory) A person or business that appears and disappears rapidly, or gives an impression of transience.

Wiktionary

A slang term used to describe someone or more than one person (can be as large as a business or any other entity) who is extremely unreliable, unless they want something from you. To put these dirtbags in their place, one must confront them directly face to face. This term is different than a flake, as a fly by night DOES contact a person, but only when they want something, whereas a flake or flaky person DOES NOT.

Urban Dictionary

"Well you got yours so fuck everyone else right?" –reddit

I'd call this person a

fair weather friend : One who is friendly, helpful, or available only when it is advantageous or convenient to be so.

I'd say:

Okay, you are talking like this because you are a fair weather friend and you feel safe and relieved now, but are you sure it was your last problem?

The following quotes (not all proverbs) are related to thankfullessness:

• No one sees how much you do for them, they only see what you don't do.

• Most people return small favors, acknowledge medium ones and repay greater ones with ingratitude. Benjamin Franklin (1706-1790).

• When our perils are past, shall our gratitude sleep? George Canning (1770-1827), British Prime Minister.

Derived from the last one, you may say:

Okay, I see that your gratitude sleeps when the perils are past. Feeling safe and relieved now, are you sure that it was your last problem ?

In an ironic manner, one may quote Stalin that said that "Gratitude is a sickness suffered by dogs".

Although the owner of the donkey has the good sense to wait until the requested help has been completed, your expression is otherwise somewhat similar to the fable (as it’s described in ‘Wikipedia’) about “The Scorpion and the Frog”, where it’s shown that a/the scorpion will inevitably and inescapably “reveal or show its true nature, its true colors”.
(from ‘The American Heritage® Dictionary of Idioms by Christine Ammer,’ via ‘The Free Dictionary by Farlex’)

“By your unkind words/actions [I guess] you’re just revealing your true colors [to me]. I’ll know better next time.”

Though not formal, you could say something to the tune of "You used me and threw me away." for criticism. Another expression related to the situation described is 'in times of need, one has to bow before the buffoon'; implying that a person is only seeking help because he/she is in dire need of it.

"Casting pearls before swine" - To give things of value to those who will not understand or appreciate it.

"No good deed goes unpunished"

"Passed along like yesterday's gossip"

"The sword has forgotten the smith that forged it."

I had to look up these English proverbs up as they've fallen out of fashion, but they speak about ingratitude.

• When I had thatched his house, he would have hurled me from the roof.
• Save a thief from the gallows and he’ll be the first to cut your throat.
• it is an ill guest that never drinks to his host

_{A compleat Collection of English Proverbs (1737) By John Ray}

However, in the situations described by the OP, I am struck more by the deceptive nature of the people than their ingratitude. I would suggest the following idioms

1. use someone for your own ends (=to get what you want)

   Liz has always used people for her own ends.

In the situations described by the OP one might tell the assisted friend, or newly wedded husband

You just used me for your own ends, didn't you?

2. take someone for a ride

   informal Deceive or cheat someone
   — it’s not pleasant to find out you’ve been taken for a ride by someone you trusted

Dont bite the hand that feeds you.