UTR number

Hi Just had a quick question for a friend she is planning to work for some one through self employed but they want the UTR number on every invoice just want to check is it okay to email this number to the person she is working for. She doesn't have a company so would be known as a sole trader otherwise they won't pay her. What are the risks? Is this normal to give UTR number to the client she will be working through self employed? They said they need it to check she is a sole trader and for payroll but she needs to give it every time she submits an invoice along with her national insurance number, name and address. Can the UTR number came into the wrong hands how is this corrected?

Hi amrt,
Normally, if self employed you would include the UTR on the invoice. 
Thank you.