Although after Pip became wealthy Pumblechook was very nice and accomodating to Pip, he is not sincere. He is just like the kid at school who sucks up to the rich kids in the hopes of being popular and getting to play with really cool toys as a result of it. Pumblechook is entirely insincere in his fond affections for Pip after Pip achieved his fortune. We know this because before the fortune landed in Pip's lap, Pumblechook lost no opportunity to harrass Pip, give him advice on how to not turn out badly, and to insult and embarrass him. In fact, Pip hated being in Pumblechook's company for that reason--he felt constantly demeaned and poked at. Pumblechook constantly tells Pip that he is an "ungrateful boy" and that he should be more mindful of his sister's attentions to him. So, before Pip's money, Pumblechook gives off the very blatant impression that he feels Pip is ungrateful, in need of much improvement, and an annoyance all around. Before the forture, Pip was very annoying indeed.
After the fortune, Pumblechook does a 180 and all of a sudden decides that he loves Pip. Besides, Pumblechook is greedy. We learn right off the bat that Pumblechook is an insincere man who is fixated on money. In some editions of the book, the publishers include a description of characters. Pumblechook's listing states that he is "a merchant obsessed with money," so naturally when Pip is found to have a lot of it, Pumblechook's first reaction is to suck up to him. He starts, for once, being nice to him, in the hopes that Pip will "be nice" in return, hopefully dispensing that kindness in a monetary fashion. He doesn't really feel Pip's fortune is "well deserved"; how could he, when the day before he was railing on Pip for not deserving anything? It is greed-induced flattery, and Pumblechook will do or say anything if he think he can get a monetary advantage--this is not sincerity, but selfishness.
I hope that those thoughts help; good luck!