what if you change the hidden element to a text field:
<input type="text" name="client" value="<?php echo $row_Activities['UserFirstName']; ?>">
does that show the value?
if so how have you set up the email body?
Did you select to create a file for the email and use one of the templates.
It would be more helpful to have a copy of the entire page rather than just the form snippet so i could see the code in context.