I am not sure about the "1 time download"
can't install the trial ver again on his system?
This can be done. try *www.softwarekey.com
This is what the publishers say
Windows 32-bit software developers can use SoftwareKey Trial Creator freeware to create fully-functional 30-day trial versions of their own software without modifying a line of source code. Potential customers will not be able to re-install the created trial software to gain time after the trial period has expired. The program is simple, easy to use, yet powerful. A 30-day trial can be created in minutes.
You can get it at *www.download.com/SoftwareKey-Trial-Creator/3000-2216_4-10287907.html?tag=lst-0-8.
Similarly you also have
DigiWrap
DigiWrap electronic software distribution wrapper (ESD). Inject real-time credit card order processing, software registration, and application unlocking into any .exe executable. Create trialware and demos (days or run count). Stop piracy with advanced software copy protection and hardware locking.
And finally
ExeShield
Hope This helps.