to Cedars-Sinai Medical Center in L.A. Friday to wish Lamar Odom happy birthday.

Lamar, who turns 36 today, is alone and laid up ... he's been there since October 19.

We're told the family will hang out in his room for a while. We're also told a select group of Lamar's longtime friends are also there. Our sources say none of Lamar's friends who are there were part of Lamar's drug crew.