Garth played Bynes' onscreen sister in sitcom What I Like About You for four years between 2002 and 2006, and admits she tried to get in touch with the Hairspray star after a series of driving offences and rumours of personal problems.
However, Garth hasn't been able to get through to Bynes to offer her support.
She tells Usmagazine.com, "I tried reaching out to her and I haven't been able to reach her. My heart feels for whatever she is going through and I love her dearly."
Bynes has been dogged by a number of driving offences in recent months, including a DUI charge and two counts of hit-and-run, and she also entered not guilty pleas to two counts of driving on a suspended licence on Monday (15Oct12).