...and I suggested long investment for him but he said he likes the short term investment because he can feed from it and I told him that, he can't invest in bitcoin and still feed from it.
There's no rule as this. You can invest in bitcoin and still feed from it especially when there's upwards movement, you can take profit and take care of yourself. So, the bitcoin belongs to the investor and the investor has the right to withdraw anytime and sort himself out.
And if he is serious I will invest for him
What do you mean by investing for him? You know it's wrong. Show him how to invest himself. It is not very difficult, don't invest for him else you take the blame when the market moves the wrong direction.