My friend has told me something about metal braces which are invisible, does such a thing really exist?

Yes they do! The 'invisible metal braces' are called Lingual appliances which were developed almost 30 years ago. The appliance needs special brackets, skills and treatment methods but the treatment result and time needed is comparable to conventional labial appliances. Because the braces are placed on the back surfaces of the teeth, the advantages of this appliance is that they are virtually invisible, However, they are more irritable to the tongue and more difficult to manage than conventional braces, so cost-wise they are more expensive.