My client has old scars. How can I best treat these?

As a rule, something like this is a medical matter about which, as beauticians, we are not allowed to make many statements, nor promises.

Of course, I understand from a cosmetic point of view that clients want to do something about this so that the scar becomes less visible. 
Without making promises to your client about this, you can try to make the scar softer and more supple.  It is then best to treat the entire area in circular motion using the Striae serum and the NBE 3000.  Place the return plates on the back at the level of the shoulder blades and work with a monopolar handpiece.  Start by gently manipulating the terminus lymph node and slide at regular intervals from the scar to behind the ear, making several circular movements there and then to the terminus with very light pressure.   In principle, the weight of the handpieces is sufficient as pressure. 
Do not work too fast and certainly not at too high an intensity: Intensity ± 35-40 % to the client's liking. It should feel like very pleasant soft warm effleurages 
  
The stretch mark serum contains phosphatidyl choline as its base and hydroxyproline on a silanol as its active ingredient. Hydroxyproline regulates the production of collagen (I) and organic silanol regulates the formation of connective tissue.  A lot of tests were conducted on this substance by Dr Pitanguy in Brazil.  Publications are available here on request, but are very technical. 
See brief description on Hydroxiprolisilane with reference to Dr Pitanguy from Brazil at the bottom of the article.

Finally, I would like to point out that this is a medical application that is generally outside our scope.   Please be sure to communicate this to the customer and explain to her that we cannot (should not) give any guarantees of success.

Publication on Hydroxyprolisilane. 
HYDROXYPROLISILANE is a multi-functional anti-aging active ingredient because it meets the skin's needs first 
HYDROXYPROLISILANE doesn't seek to meet only one of your skin's needs, but many of them! The skin can thus handle any situation. 
The combination of the organic silicone core and hydroxyproline, an amino acid and collagen precursor, makes HYDROXYPROLISILANE a multi-functional active ingredient which maximises strengthening and regeneration of the dermis. Tested (in vitro and in vivo) and approved worldwide, it is appropriate for all types of cosmetic products: cosmetics, dermocosmetics. 
HYDROXYPROLISILANE is a benchmark for combating stretch marks and dermo-cosmetics

   
HYDROXYPROLISILANE has been the subject of many studies, including one in France on groups of pregnant women, aimed at assessing the impact of treatment on the appearance of stretch marks. 
HYDROXYPROLISILANE approved by medicine and plastic surgery 
The world expert in plastic surgery and cosmetic medicine, Dr Pitanguy, from Sao Paolo, Brazil, decided to assess the effectiveness of HYDROXYPROLISILANE on optimizing skin healing. The study protocol included a course of SILANOL before cosmetic treatment and demonstrated very clear improvements in terms of speed and quality of healing. 
The hydroxyproline in HYDROXYPROLISILANE is one of the main amino acids that make up collagen.

   
Hydroxyproline and hydroxylysine are the only two amino acids that stabilize collagen’s alpha helix by securing its twisted form and its elasticity.