After a bit of research I see this was a problem with the Shield. Most definitely you should contact S&W. They are aware of the situation and appear to be offering free repairs.

Jim, Smith and Wesson should advise or fix this for you at (USA) 1-800-331-0852
Mon-Fri 8:00AM-6:00PM Eastern Time.

Does it doe it with more than one mag if so it sounds like a bad mag catch or spring and he will need to call S&W By Telephone:
(USA) 1-800-331-0852
Mon-Fri 8:00AM-6:00PM Eastern Time
(International) 1-413-781-8300
Mon-Fri 8:00AM-5:00PM Eastern Time
By Fax:
1-413-747-3317 (USA)
1-413-731-8980 (International)

Just having dealt with my own 9mm Shield I haven't had any problems. I would suggest you contact S&W and just ask them. If it is a common problem they will tell you and will probably recommend a fix. They're good about their warranties and may well offer you the chance to send it in for evaluation. I would sure ask at the least. Dropping a mag is definitely not a good thing!