I don't know enough about the flow of your application to be able to tell you _how_ to do this. there are a gazillion ways it _can_ be done.
I would suspect that your user logs in, and they see a listing of clients that they can update.
I'm going to assume that this client listing page is named "clients_Results.php"
That listing of clients has links that go to the detail page:
<a href="clients_Detail.php?ClientID=<Id from recordset>"><Client name from recordset></a>
If I am wrong on any point the advice I give will be wrong, but here is one way to do it.
change the links to point to the client results page instead of the detail page:
<a href="clients_Results.php?ClientID=<Id from recordset>"><Client name from recordset></a>
Create a recordset that is filtered on the UserID session variable and the client ID Querystring variable.
this is a lookup recordset that will be used to ensure that the clientID querystring variable is client ID that belongs to the logged in user and does not belong to another user.
Add a set session value server behavior. On the Server Behaviors panel, click the plus button and select eCart -> Genera; -> Set Session Value
Set the name to ClentID
set the trigger to Recordset <Recordset we just created name> Not empty
For the value, click the lightning bolt and select the Client ID from the recordset we just created.
next add a server redirect to the client details page. On the Server Behaviors panel, click the plus button and select WA Utilities - > Server Redirect or webassist -> Utilities -> Server Redirect
Set the trigger to Recordset <Recordset we just created name> Not empty and select the clients_Detail.php page.
