A friend who always complains

The canon's five ways to put down resentment include love, compassion, and equanimity for the person, and also, frankly, not paying attention to them — which is permission to limit what you take in without hating anyone. The canon also gives a test for speech that works both ways: is it true, is it useful, is this the time. A friend who only ever vents may need to hear, gently, that you cannot help this way. The suttas will not tell you whether the friendship should continue.

What the suttas say

“Mendicants, a mendicant should use these five ways to completely get rid of resentment when it has arisen toward anyone. What five? You should develop love for an individual you resent. That’s how to get rid of resentment for that individual. You should develop compassion for an individual you resent. … You should develop equanimity for an individual you resent. … You should disregard an individual you resent, paying no attention to them. … You should focus on the ownership of deeds regarding that individual: ‘This venerable is the owner of their deeds and heir to their deeds. Deeds are their womb, their relative, and their refuge. They shall be the heir of whatever deeds they do, whether good or bad.’ That’s how to get rid of resentment for that individual. A mendicant should use these five ways to completely get rid of resentment when it has arisen toward anyone.”
A practical list — love, compassion, equanimity, not attending, reflecting on kamma.
Where this stops: Reflecting that people own their deeds is never a way to call someone's misfortune their kamma, and putting down resentment is never a reason to stay near someone who keeps doing harm.
AN 5.161:1.1–1.15 · Getting Rid of Resentment (1st)
Even as a mother would protect with her life her child, her only child, so too for all creatures unfold a boundless heart. With love for the whole world, unfold a boundless heart: above, below, all round, unconstricted, without enmity or foe. When standing, walking, sitting, or lying down while yet unweary, keep this ever in mind; for this, they say, is a divine meditation in this life.
The Metta Sutta's central image, in the canon's own words.
Where this stops: The verses are an ideal of the practitioner; the opening lines describing a skilled and honest person are a description, not a prerequisite for beginning. Never use it to quiet anger that has something to say, and never present it as a recitation that protects anyone. The mother guarding her only child is not for someone after a pregnancy loss.
Kp 9:7.1–9.4 · The Discourse on Love
“Do you then serve them with fresh and cooked foods and savories?” “Sometimes I do.” “But if they don’t accept it, brahmin, who does it belong to?” “In that case it still belongs to me.” In the same way, brahmin, when you abuse, harass, and attack us who do not abuse, harass, and attack, we don’t accept it. It still belongs to you, brahmin, it still belongs to you! Someone who, when abused, harassed, and attacked, abuses, harasses, and attacks in return is said to eat the food and have a reaction to it. But we neither eat your food nor do we have a reaction to it.
Declining to take in what someone hurls at you in anger, the way a guest can leave a dish untouched.
Where this stops: Concerns how abuse is received inwardly; it is no reason to stay where someone keeps abusing you.
SN 7.2:2.5–3.2 · With Bhāradvāja the Rude
We will meditate spreading a heart of love to that individual. And with them as a basis, we will meditate spreading a heart full of love to everyone in the world—abundant, expansive, limitless, free of enmity and ill will.’
Mettā starts with the specific person who spoke harshly, then widens; the order matters.
Where this stops: A training for one's own mind under hard speech. It is never a reason to stay where someone keeps speaking to you with contempt or threats, and never a ruling on whether to leave.
MN 21:11.14 · The Simile of the Saw
“In the same way, prince, the Realized One does not utter speech that he knows to be untrue, false, and pointless, and which is disliked by others. The Realized One does not utter speech that he knows to be true and correct, but which is harmful and disliked by others. The Realized One knows the right time to speak so as to explain what he knows to be true, correct, and beneficial, but which is disliked by others. The Realized One does not utter speech that he knows to be untrue, false, and pointless, but which is liked by others. The Realized One does not utter speech that he knows to be true and correct, but which is harmful, even if it is liked by others. The Realized One knows the right time to speak so as to explain what he knows to be true, correct, and beneficial, and which is liked by others. Why is that? Because the Realized One has sympathy for sentient beings.”
Prince Abhaya — the test for whether and when to say a hard thing.
Where this stops: For deciding whether and when to say one particular hard thing; it does not judge a relationship as a whole, and it is not a reason to say nothing. Nor does it mean anyone owes another person news about their own body, health or pregnancy, or should speak when speaking would put them at risk.
MN 58:8.1–8.8 · With Prince Abhaya

Bring your own situation.