Idiom request for recommending someone to end their toxic relationship/ friendship with somebody

Consider to cut somebody loose

to get rid of or release someone or something

[The Free Dictionary]

USAGE

The husband cut loose his nagging wife.

Posting from Mobile web during travel. Excuse formating

You might say,

She/he isn't your cross to bear. Ditch him/her/Cut all ties with him/her now or you might regret it for the rest of your life.

bear one's cross and carry one's cross

Fig. to handle or cope with one's burden; to endure one's difficulties. (This is a biblical theme. It is always used figuratively except in the biblical context.) It's a very painful disease, but I'll bear my cross. I can't help you with it. You'll just have to carry your own cross. McGraw-Hill Dictionary of American Idioms and Phrasal Verbs

ditch

: to stop having or using (something you no longer want or need) : to get rid of (something)

: to end a relationship with (someone)

: to get away from (someone you do not want to be with) without saying that you are leaving

(Merriam-Webster)

cut/break (all) ties with (someone)

To end or discontinue a relationship—romantic or otherwise—with someone or some group. Mary cut all ties with her family when she moved to New York City. The government began cutting ties with the war-torn country after its human rights atrocities came to light. Farlex Dictionary of Idioms

[leave] that guy/girl before he/she spells trouble for you

spell trouble

To be the cause of possible problems in the future (often + for)

Cambridge Idioms Dictionary, 2nd ed.

One I have heard often (though perhaps not often enough to be an idiom) is "you are not required to set yourself on fire to keep others warm".

So in this case, setting yourself on fire would be to stay with the toxic partner.

Quit beating your head against the wall.

beat one's head against the wall and bang one's head against a brick wall Fig. to waste one's time trying hard to accomplish something that is completely hopeless. You're wasting your time trying to figure this puzzle out. You're just beating your head against the wall. You're banging your head against a brick wall trying to get that dog to behave properly.

beat head against the wall. (n.d.) McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. (2002). Retrieved March 4 2016 from http://idioms.thefreedictionary.com/beat+head+against+the+wall

It's not specific to relationships, and it's about futility more so than pain or toxicity.

Cut the cord means to quit supporting someone else (or to stop receiving support). But it doesn't imply the relationship is harmful. It would need additional context.

cut the (umbilical) cord to end support of someone or something, esp. financial support He needs to cut the umbilical cord, get away and find his own place in the world. By criticizing his party so strongly, he cut the cord and now has to raise campaign money on his own.

cut the cord. (n.d.) Cambridge Dictionary of American Idioms. (2006). Retrieved March 4 2016 from http://idioms.thefreedictionary.com/cut+the+cord