The Twilight star stunned Hollywood last month (Jul12) after admitting to an "indiscretion" with married director Rupert Sanders.
The revelation has turned her into a target for bitter fans and members of the press alike, who have dubbed her a "trampire" for betraying her boyfriend Robert Pattinson.
Foster, a former child star herself, sympathises with Stewart, who played her daughter in Panic Room, and insists she should not have to deal with the intrusive media.
In a lengthy article posted on news website TheDailyBeast.com, Foster recalls the moment she met a happy, sociable 11-year-old Stewart on the set of 2002 film Panic Room, admitting, "I grew to love that kid."
And she fears the stresses and strains of life in the spotlight have taken their toll on the actress.
Foster adds, "A beautiful young woman strides down the sidewalk alone, head down, hands drawn into fists. She's walking fast, darting around huge men with black cameras thrusting at her mouth and chest. 'Kristen, how do you feel?' 'Smile Kris!' 'Hey, hey, did you get her?' 'I got her. I got her!' The young woman doesn't cry. F**k no. She doesn't look up. She's learned. She keeps her head down, her shades on, fists in her pockets. Don't speak. Don't look. Don't cry."
Wrapping up her commentary piece, the Hollywood veteran urges Stewart to fight through the tough times - because surviving a media storm and coming back stronger is the best way to overcome adversity.
She continues, "Eventually this all passes. The public horrors of today eventually blow away. And yes, you are changed by the awful wake of reckoning they leave behind. You trust less. You calculate your steps. You survive.
"Hopefully in the process you don't lose your ability to throw your arms in the air again and spin in wild abandon. That is the ultimate F.U. and - finally - the most beautiful survival tool of all. Don't let them take that away from you."