The 23-year-old recently underwent ten plastic surgery procedures in one day, including a mini-brow lift, breast augmentation and liposuction.
Speaking to ET Online, Cavallari said: "I've seen the pictures. Who are we to say what she can and can't do? We're not the ones that have to live in that body.
"If it makes her happy, then go ahead. It's not my body... Do I think she's very young? Yes. I'm 23 and she's 23, it's very young to be getting that amount of work done. Would I do it? No. But it's not me."
Cavallari went on to say that she was not in a position to "judge" her co-star, saying: "If it makes her happy then I think it's fine. It's not me. I don't have to deal with it."
She added: "I've known Heidi since she was 18 and I've always thought she was beautiful, so it's hard for me to see this transformation that's taken place, but I love Heidi and she still has the same bubbly, sweet personality that I see."
Other Hills stars to have spoken out about Montag's surgery include Audrina Patridge, Lo Bosworth and Jayde Nicole.
> Montag's mother 'knew about surgery'