Why had Shylock's friend Tubal been to Genoa ?
In Act 3, Scene 1 of Merchant, Shakespeare does imply through Shylock and Tubal's conversation that Shylock sent Tubal after his daughter. When Shylock sees his fellow Jew, the first words he speaks are:
" 'How now, Tubal, what news from Genoa? hast thou / found my daughter?' " (3.1.70-71).
Tubal responds that he came close to finding her but never actually did. His immediate response implies that he was sent to Genoa and meets with Shylock in this scene to report back to him. In addition to telling Shylock the "happy" news that Antonio is most likely going to have to forfeit his agreement, Tubal also lets Shylock know that his daughter has traded a sentimental ring that belonged to Shylock as if it were nothing. This is important because it relates to one of the play's subplots (the ring subplot--see Acts 4 and 5 regarding the ring storyline involving Bassanio and Portia and Gratiano and Nerissa). Rings serve as a motif in this play and represent the value that one character places on another--if a person gives away a loved one's ring easily, Shakespeare seems to imply that that person might be fickle with his or her love or loyalty.
As you say, Tubal is one of Shylock's friends, who brings him news from Genoa.
We do not really have any clear idea about why Tubal was in Genoa. We can assume that he has been there to do some kind of business. However, that is not the subject that he talks to Shylock about.
All that he and Shylock talk about is what Tubal has found out in Genoa. He does not really know much about what has happened to Shylock's daughter. All he knows for sure is that she's spening money -- doing things like exchanging a ring for a monkey.
What Tubal tells Shylock that makes Shylock happy, however, has to do with Antonio. He tells Shylock that Antonio's ships have sunk and Antonio is going to be bankrupt.