Is there proof of service (other than a DD 214) for Reservists?

You could pull his retirement point statement. There is something in the works to serve as a 214 for the USAR, but not sure how far along it is in the process before it is fielded.

As others have stated, there currently isn't a single form that is similar to a DD-214 or NGB-22 for reservists. Each branch under DOD is required to complete a DD-214 for any Title 10 active duty that is more than 90 consecutive days or for active duty for training for initial entry training. Many are created for shorter periods, but are not required. I'm not certain of Title 32 active duty, but would assume it is comparable. Here are the forms used by the different branches that show retirement points and can be used to demonstrate total service by themselves or along with a DD-256.

Branch Type of Form
Air Force Reserve Record of Service AF 526, Point Summary Sheet
Air National Guard NGB Form 22, Report of Separation & Record of Service
Army Reserve ARPC Form 249-E or DARP Form 249 or 249-2-E, Chronological
Statement of Retirement Points
Army National Guard NGB Form 22, Report of Separation & Record of Service
Navy Reserve NRPC 1070-124, Annual Retirement Point Record
USMC Reserve NAVMC 798, Reserve Retirement Credit Report
Coast Guard Reserve CG 4175, USCG Reserve Retirement Points Statement

A DD 214 is a "Certificate of Release or Discharge from Active Duty". A Reservist is not being released from Active Duty, so they don't get a DD 214. If they deployed or any had any other periods of Active duty status for over 90 days, then they'd get a DD 214 (as well as one from finishing basic and AIT) that will cover all their Active Duty time - not drill weekends and such, but Reservists do not get one for ETSing, only Active Component and AGR personnel do. When a Reservist ETSes, they receive a set of orders releasing them from their service, just like Active Component personnel, that they use to clear their Reserve unit, that serves as proof of discharge. If they are looking for proof of how long and all that, retirement points are the best bet. They could ask their unit for a memorandum for record that summarizes their service for job applications and stuff like that.