|Rumer Willis and Demi Moore|
Zac - who was recently romantically linked to Rumer - told US TV show 'Extra': "It's tough, but I think she's all right.
"Me and Rumer have been friends for a long time. I know Demi is going through some personal stuff right now. I just wish her well. I hope she's doing OK."
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Meanwhile, despite multiple reports stating Demi is currently receiving treatment at the Cirque Lodge clinic in Utah, several sources have told gossip website TMZ that this is not the case, though it has not been revealed where she is at present.
Last week, Demi's three daughters, Scout, 20, Tallulah, 16, and 23-year-old Rumer, as well as their father Bruce Willis, were said to have encouraged the 'Striptease' star to seek professional help, but she refused to go to rehab.
Her estranged husband Ashton Kutcher - who she split with in November amid allegations he had cheated on her - is also thought to have urged Demi to go to rehab following her collapse.