Legal question

I was contacted a couple of months ago by the owner of a property management company to do a landscape maintenance job at a residential home for around $400. The owner of this company also asked me to give him two more estimates for a different residential customer. He paid me by check in mail after completion of first job so I figured he was probably going to be good for the two jobs at the next place, which came out to be about $1000 in work. He wasn't good for it, and I've contacted him on several occasions and only once did he return an email stating that the check was in the mail and that was about a week and a half ago. So my question is, can I legally contact the customer at the residence of the $1000 in work that I wasn't paid for and kindly ask for a letter stating that the work was completed to their satisfaction, to use in court? And also, can I legally send a letter to the first customer who I was paid for, to make them aware that they're paying this guy and he may not be paying their contractors? Thanks!