1. check the acs_mail_lite_queue and make sure there is no message in there that is being sent but not deleted. if there is one in there that is being sent and re-sent you could manually delete it
2. change this users email address to another one (possibly yours) and check that email address to see if messages are now routed there (in which case you can be pretty sure it is in fact openacs that is sending the mssages). This way you will get the annoying messages instead of your client and this makes it less stressful to figure out the problem.
3. if you've done step two and if that didn't fix it you can do a full pg_dump of the database then grep for that users email address. This will let you find any other tables that might contain this persons email address in openacs - which you can then manually update to your address with psql.
If this persons email address is NOT in the pg_dump, and still getting email messages, after a restart of postgresql and aolserver then there is clearly something wrong with your mail server setup, and that's something different to tackel, at least we will know what to look into... I have had mail stuck in the queue with sendmail, postfix and qmail (i haven't used other MTA's), I am guessing this is the problem, but the above steps should help us isolate where the problem is... in all seriousness, it COULD be that this users email service is having problems, and not your server, but I wouldn't tell them that unless you are sure its the case...