I can't help you with the best time to call, as I have found it varies greatly, but I can tell you if the check was cashed at the end of March they definitely should have gotten their passport by now. Did they try tracking it online? They/ you can do so at
Checking the Status of Your Application Even with the new processing times it still should take no more then 10 weeks for them to receive their passports. All of my clients have gotten their passports much sooner then that.
Sorry I couldn't be of more help.