Bynes was pictured snoozing on a couch at the Beverly Center in Los Angeles on Thursday morning (06Nov14), just days after she was released from a psychiatric hospital.
She later went online to accuse her parents of using their conservatorship to withhold her money so she could not afford to pay for a hotel.
Former The Hills star Montag was so disturbed to hear of Bynes' troubles, she reached out via Twitter.com to offer the actress a room at her home in Santa Barbara, California.
Montag wrote in a message to Bynes, "You are more then welcome to crash in our guest house in Santa Barbara if you need somewhere to stay! Dm (direct message) me!"
The Hairspray star was released from a psychiatric hospital last week (ends02Nov14) after being committed involuntarily for 17 days. She later claimed to have been diagnosed with bipolar disorder. Her parents have been given control of her finances and Bynes alleges they have been restricting her spending to $50 (£31) a day.