2 yrs is far too long a time for a guy to expect you to vouch for him on a verification site.  If a guy wants a ref from you on one of those sites, he should make it known immediately after the session with you, not 2 years later.......duhhhhhhh, c'mon guys.

You could tell them if they want you to reference them, they need to pay you another visit to refresh your memory, or something like that.  I do suggest you be honest with them in telling them that you cannot reference them now.  When we write reviews, they have to be within 90 days of the date or they are not approved by TER.  However, for a provider I would recommend a much shorter time period in which a client can ask for a verification from you.

Swim

If that much time has gone by -- and its something that you can't remember....then dont vouch for them.  I think what most people are looking for is simply a thumbs up that you met with them and they were good people...not creepy...seemed sane....didnt give you a negative vibe...and the session went nicely.  It's the same as someone reviewing you -- even on TER it only allows you to review a person once no matter how many times theyve seen you.  So all of those single visit stories counts towards your popularity and trust as much as their single provider validations.  Thats my 2c.  But yeah -- someone crawling out of the woodworks many months later -- no thumbs up.