What lesson might act 2 give about following the advice of others?

In act 2, Roderigo continues to take advice from the machiavellian Iago, unaware that he is being used by Iago to bring about Othello's demise. Iago convinces Roderigo—who is desperately, naïvely, and hopelessly in love with Desdemona—that once Desdemona is tired of Othello, she will be courted by Cassio. Therefore, Iago advises Roderigo to "find some occasion to anger Cassio" so that Cassio, Othello's lieutenant, will lash out in response and then be "profitably removed" by Othello as punishment. Roderigo unwittingly agrees, stating, "I will do this, if I can bring it to any opportunity." Iago's plan works out as far as Cassio is concerned, as Cassio lashes out when provoked by Roderigo and is duly cast out of Othello's favor. This turn of events is the first part of Iago's plan, which comes to fruition at the end of the play with the death of Othello. Therefore, by following Iago's advice, Rodrigo unwittingly has a small part to play in the unfolding tragedy to come.
There are a number of possible lessons here about taking advice. The first is that one should be careful as to who one takes advice from. Although to be fair to Roderigo, he is certainly not the only one guilty of misjudging and taking disingenuous advice from Iago. The second lesson is perhaps to be wary of taking advice pertaining to your love life when you are too naïvely and desperately in love to discern the quality of that advice. Roderigo also pays handsomely for Iago's help, thinking that the money is going towards a plan to secure the love of Desdemona. Iago constantly urges him to "put money in thy purse" to receive his advice in return. Perhaps a third lesson then is to be wary of advice that you pay for, especially when the continuation of the payments depends on the advice not leading to a successful conclusion.
In act 2, scene 3, Iago also offers his advice to Cassio, who by this point has already been dismissed as an officer by Othello because of Iago's advice to Roderigo. Iago advises Cassio to go to Desdemona and "confess [him]self freely to her" so that she might entreat Othello—on Cassio's behalf—to reinstate him. Iago's advice is once again for his own benefit only. He hopes to drive Othello mad with jealousy by having him happen upon Cassio and Desdemona together and by then convincing him that something untoward is going on between the two of them. In response to Iago's advice, Cassio says, "You advise me well." The audience will be acutely aware of the irony here. Indeed, in his soliloquy a few moments later, Iago professes that his advice is intended only to "make the net / That will enmesh them all."
The lesson about taking advice that we might garner from Iago's interactions with Cassio is perhaps that one should be skeptical of taking advice from one who stands to profit if that advice fails. Cassio would likely be aware that Iago was hoping to be promoted to the position of Othello's lieutenant ("I am worth no worse a place") and should thus have been able to reason that perhaps Iago might be resentful of him. Iago stands to be appointed as Othello's lieutenant (from which position he can more effectively engineer Othello's demise) if Cassio is permanently cast out of Othello's favor. Cassio, however, is somewhat naïve, and takes Iago at face value. Whether this is the fault of Cassio for being naïve or Iago for being so machiavellian is another debate entirely.