How can gold be traded to other players Diablo 3?

You can't trade gold. It is no longer possible to transfer gold between accounts in any way or form.

From patch 2.0.1 notes:

Gold can no longer be traded

It may be possible to purchase items from the merchants and giving it to him to sell, but this will be at a massive loss rate.

If you wanted to help him along, there are still several ways to assist in his gold income:

• Take him along while you do bounties. Each completed bounty earns a fixed amount of gold (based on difficulty level).
• Take him along for rifts. The rift boss usually drops ~ 50 blood shards, which can be used to purchase 10 pieces of armor at the gambler, each selling for somewhere around 3-5k a pop. This can add up quite fast. You also earn some gold completing the rift quest.
• Give him all the rares you find as you play. The rare sell values has really gone up in RoS and making a few million doesn't take long at all.