1) Can you use Maaser money for a wedding gift (simchas chosson vekaloh)?
2) If yes, could you pay for a flight to visit a very good friend’s wedding with maaser money?
If you would have given the gift even without Maaser or you are expected to give a gift, maaser should not be used. The same is true with flying out, if it will bring them joy and without maaser you would not have gone, maaser money may be used. This is assuming you have not established to give your maaser money exclusively to poor people.