hi Lee,
you will have to use conditional formatting for this.
before you send the email, set a value(say customercopy) to some value (say yes).
on the fields you want to be read only to the customer.
right click the field and click conditional formatting and check the value of customercopy field. If it is equal to yes set the fields readonly.
another easier way will be to create another view name it as customerview, set the fields read only which ever you want. on form opening switch the view based on value in customercopy.