It is hard to believe that they cannot remember their email address. All they have to do if they really cant remember is to check all the emails they have for anything from your site domain by doing a email search. If they find something then that is probably the email they used, if not then it is not.
You have to protect yourself as a site owner, people have and will come up with wild stories to get someones account access. Be firm and follow your own site policy and do not give them the information.
You cannot give them their password anyway, you do not have access to it unless you know how to decrypt a hash from the db.
What you could do if you really wanted to, is create an account so that you know the password. Then take that password from the password field in the db and copy it over to their account, then you give them that pw that you know. Then delete that account you just made. However there is such a thing as member responsibility here and you have to force the members to take you and your site seriously that you just do not hand out information like that.
Someone might be testing you as well to see how firm you stand on security and then come back at you if you give it out, don't fall for it. If they are real and true then they can dig through their emails or brain and try to remember the email address and then use the password reset process.
There used to be a plugin that would let admin change passwords but i do not know if it still works or not and i do not remember the name of it.
But still stand firm. They have to share the responsibility. If i went to the bank and said i lost my information and cried in their arms trying to get them to give it to me they would tell me i have to follow the process. So you need to do the same or you will get burned one day.
Dave