I checked the interline agreement that United has for travel on JetBlue - it does state that the $25 non-rev ticket needs to be issued by JetBlue at their ticket counter, and what they require is a letter from a direct supervisor dated within 90 days stating seniority date, employee number, and all of the employee's travel eligibles. This is different from most of the agreements that United has which usually require United to issue the tickets for its own employees. It may just be the way JetBlue does business, so check the specific interline agreement they have with your airline to see what it says.