Buy a Roblox Gift Card
Send the gift card code to the person
Redeem the gift card on Roblox
Use the Robux or Roblox credit from the redeemed card
Buy Robux directly from their Roblox account if you have access and permission
Purchase a Roblox Premium subscription for them if you want to give them monthly Robux
Use the Roblox group payout system if they are in your group and you have Robux to distribute
Trade limited items for Robux only if both accounts meet Roblox trading requirements