There s no way he can ROI in 5 months with S5. With static diff, ROI is 260 days. With diff increase over 300 days at best.
i think your calculator is broken, one s5 still do 0.01 a day(actually is more... 0.011, the diff dropped a bit), in 5 months your earning is 1.5 btc, with 1.5 btc you can buy 1 used antminers s5 here on the forum, if you want it new then you need 2 btc, and the roi will be extended a bit, but no way it is 260 days
this is assuming that the diff won't rise of course
I would say your calculation s bit off. S5 can do 0.011, even above that, but you have to pay maintenance fee which reduces the dividend.
Take a look at this:
http://nextdifficulty.com/It gets updated often so the number can vary a bit but that s it in essence.
Also, last diff went up 2.44% and the next one ll go up as well as it seems ATM, probably even more then that.
Mining s a very difficult business, constant reinvesting is required. This is why I think our newbie friend s gonna waste his hard earned money if he s not careful.