If they say they didn't register it or "don't remember" where then chances are they're lying and/or are a fake. Your DD214 is your ticket to your VA benefits and only a fool (or someone with a dishonorable discharge) doesn't register it.
If they did register it, ask them to have the registering office send you a certified copy. This way you will know it is legitimate since it will be signed by that office. Note that there is usually a small fee for this but it's worth it.
You can request info through the FIA but be prepared to wait a couple of months minimum for it. I can't even get copies of my own records any quicker!
Some private agencies may be able to do a verification of military service for you for a smaller fee than a full background check. Call and ask- it never hurts.
The other way would be to try Classmates and see if anyone that was stationed with them remembers them and if what they say jives with the person said.
A lot of military commands are on there now and many ships/stations have their own society that can also help you find out if someone was really stationed there.
Hope this helps.