you need to put some form of activity on the account which will extend the expiration date. Mileage always expires a certain amount of time since the last activity posted to the account. If he can, have him get a delta Skymiles Amex card and spend a few $'s which will also give him the bonus which would probably be considered activity (you can do the same with the Citi AAdvantage card)- if he cant/wont get the Amex card you need to post some activity either bu flowers 1800-flowers see this link
http://www.delta.com/skymiles/index.jsp you can get mileage for it or you can use some miles to sign up for a magazine subscription, you can find this on the Delta website under redeeming miles. the cheapest magazine subscription starts at 400 miles.
Hope this helps.