Gomez opened up earlier this month that she had undergone chemotherapy to treat the illness when she was rumored having addiction to drugs.
The "What Do You Mean?" hitmaker pampered her during her hard time dealing with the illness, reported Aceshowbiz.
The 23-year-old songstress was afraid before beginning treatment, particularly because she might lose her hair, but "she had Justin to support her through the whole ordeal."
Detailing the actions that Bieber, 21, would take to show his support for Gomez, the source said, "He would always tell her she's the most beautiful girl in the world, and that it wouldn't matter if she gained weight, lost her hair, or anything."
The couple was dating on and off before they broke up last year.
"They have obviously had a lot of ups and downs in their relationship, but he really stepped up for her," the source says of the exes, adding, "That's not to say there haven't been plenty of other times when he was a jerk, but he knew she was struggling, and he wanted to let her know he was there for her.
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