It is true that if a friend buy something new, I should congratulate him and tell that it was a good decision just to make him feel good? Even if a disagree with that? Or I should tell him the truth about what I'm thinking?
Answer:
You should congratulate him, and tell him how good the purchase is, even when it really isn't that great -- but make sure you sound sincere.
The source for this is a Gemara in Kesubos (17a), which cites the words of Beis Hillel assuming that when a person makes a bad purchase from the market, one should certainly praise it, rather than telling the truth.
This is also the halachic ruling with regard to praising a bride on her wedding day.
