He's probably heard that when you are young you tend to recover from a cycle quicker. That's true, but in this case the "young" guys that are bragging about quick recovery are in their 20's not 15yo. At his, and your age, your body is producing crazy amounts of natural test for all kinds of important reasons.
That test is being converted to DHT, estrogen and other hormones for other important uses (sexual maturity, libido, bone growth regulation, probably a dozen other functions). This isnt a process you want to interfere with, there's a delicate ballance in your body that's important today AND is an important building block for your future ability to regulate hormone production and overall health. Deca is a great way to shutdown this natural test production.
Starting a second cycle without even bothering to recover is just plain stupid. Ask him why he started the second cycle. The reason was that he started loosing mass too fast and couldn't stand the idea. Ask him how that situation is going to be fixed by waiting another 10wk to quit this new cycle. The problems still waiting for him, getting worse every day he stays on deca.