Joint mortgage but smaller deposit?

My little sister is looking to buy her first flat and is considering buying with a friend and getting a joint mortgage so that they can borrow more. She has been saving for a couple of years and almost has a deposit but her friend has no savings just yet. When taking out a mortgage is it generally assumed that the deposit is shared 50/50 like it might typically be with a couple? Would she need to get something in writing from a solicitor so that she could show that she had put in more money when they come to sell the flat one day?

Thanks for any advice, P :)