I have a tenant paying late and behind in his rent. He's
Been evasive and unresponsive. I called his home number
And got a hold
Of his roommate/girlfriend who is not on the lease. She
Said she would send money. I got the check but the memo
Line says, "Rent for Cheryl Smith." Should I cash it?
Eviction is not to far off for both of them. This is in
California.

If you have ever accepted payment from the roommate, that person became a tenant who has rights.
If not, it would be best to send it back (possibly with a rental application) and explain that you can only accept rent from the leaseholder, and start eviction against him.
It could get sticky since it's CA and you have 'dealt' in a way with her.