CHAZ BONO is in the middle of a pal's heartbreaking breast cancer battle.

Cher’s bighearted son has a close friend, Cathy Williams, who’s battling the dread­ed disease – and he’s moved her into his modest West Hollywood home so they can face the life-and-death struggle together.

“Chaz is amazing!” Cathy told The ENQUIRER outside the home. “He’s been a lifesaver.

“We’ve known each other for years. I am a veterinarian, and I used to care for Chaz’s cats. I also rented his guesthouse for a while. We have always been very close friends.

“I’ve been battling breast can­cer and had both of my breasts removed. It was anguishing and obviously I’m very emotional.

“Chaz didn’t hesitate to help me. I’m staying with him until I feel stronger. I walk his dogs, and we keep each other company, and I am eternally grateful to him.”

Another source says that Cathy’s potentially deadly illness brings back bittersweet memories of Chaz’s former girlfriend, Joan Ste­phens, who died of non-Hodgkin’s lymphoma when she was 47.

“Chaz had tears in his eyes when he asked Cathy to move in because he cares about her so much,” said a close source. “This is one of the most difficult things he has ever done because it reminds him of Joan.”

As The ENQUIRER reported in 1994, Chaz, 44, had a torrid affair with Joan – who was 23 years his senior – while he was still living as “Chastity.” He underwent female-to-male reassignment surgery in 2009.

“Joan was a close friend of Cher and served as a baby sitter for her two children, Chastity and Elijah Blue,” an insider later told us. “Years later, a lesbian love affair between Chastity and Joan bloomed, and they moved in together. When Joan got sick, Chastity became her caregiver till the end.”

Sadly, Joan died in 1994.

“Chaz and Joan were deeply in love,” the source continued. “So Cathy’s cancer battle has brought back painful memories for Chaz, but he’s determined to help her live.

“While he and Cathy have never been lovers, he still cares about her. And in the memory of Joan, he’s helping this woman through her battle by playing nurse.”

But Cathy told The ENQUIRER that she’s also helping Chaz during a lonely time in his life.

“Chaz doesn’t have a girlfriend right now,” said Cathy. “He has helped me so much, but hopefully I am helping him through a time in his life when he has no one.

“We’re both here for each other.”