You cannot directly send Robux to a friend
Buy a Roblox Gift Card and give the code to your friend
Join the same Roblox group and use group payouts if you own the group and have Robux in the group funds
Buy your friend a game pass or item they created, if you want to support them
Use the Roblox Premium Payouts system if you are both eligible and the game supports it
Be careful of scams and never share your account password or verification codes