D4vd's former friend Aysia Collins claims she confronted the singer about his alleged sexual relationship with 13-year-old Celeste Rivas ... and she says he manipulated her into believing Celeste was an adult.

Aysia -- who ran in the same group as the singer and the slain teenager -- says she initially did not take D4vd at his word ... and pushed him for proof when he insisted she was 19.

She says D4vd provided her "proof" -- what that was, she never says -- and then she started to back off, feeling she was "harassing a friend" by pressing the issue.

As you know ... Celeste's decomposing and dismembered body was found in D4vd's Tesla back in September ... and he's charged with one count each of murder, continuous sexual abuse of a child under the age of 14 and unlawful mutilation of human remains.

Back in June, Aysia called D4vd a "psychopath" who "lied and betrayed everyone around him."

Another friend of D4vd's, Neo Langston, also distanced himself from the singer in the months since Celeste's body was found.

D4vd has pleaded not guilty.