Tickets are not necessary for an application and best practice is that they should not be purchased in advance of a visa decision. Priority service is generally reserved for those with a clean application history, and you mention an earlier refusal.
Applications with a prior refusal are considered complex and are directed into the 'red' queue, regardless of whether priority service was requested. When this happens, the applicant is not reimbursed for the difference in cost, and the advertised turn-around time is no longer in effect.
Whether the visa would be refused for an insufficient amount of time would depend on the application's premise and the extent to which the applicant's bona fides have been established. Fortunately, some posts will just reject the application rather than issue a refusal. Hopefully, that will be the case, so that there will not be a second refusal on his record.
You can use this UK.gov tool to obtain information on how to contact UK Visas and Immigration about the application.