A source said: "Russell and Charlie have been spending an increased amount of time together as they've been named by [TV network] FX as the stars for their summer schedule so have done a lot of promotional work together.
"Russell has invited Charlie to join him at his yoga classes.
"But Hollywood is a small place and everyone is worried Charlie might lead Russell astray.
"Russell is a strong guy, he won't just fall off the wagon, but he's single after his split from Katy Perry and on the look-out for women - and Charlie's legendary parties are always packed full of beautiful women."
Despite warnings from his friends, Russell is always happy to hang out with Charlie - who was sacked from sitcom 'Two and a Half Men' last year because of his erratic behaviour and wild partying - as he knows what it is like to go "off the rails".
The source added to the Daily Star newspaper: "Russell's friends are panicking as Charlie keeps calling him, chasing him to meet up.
"Russell is too kind to say no, he thinks Charlie just needs a friend, someone who'll look out for him. Russell was once in the same situation, off the rails and lonely.
"But Charlie is a notorious bad boy and will definitely be a bad influence on him."