Mining isn't my thing but judging by the stack of S3+ and S4 units that a friend has asked me to sell for him I assumed that the S5 was not too far off...
Most likely your mate wants to sell because profit is not worth the noises after cost of electricity at current BTC price. I'm using my S1s, S3s and S4 as space heaters and make little profit. My flat is so hot that I have to keep my windows open.
Either your electric cost per kwh is high or your pool is not paying enough because I am making profit using S3's and S4's. $.12 a kwh electric and Slush pool. Winning combination.