You cannot directly gift Robux to a friend through Roblox
Buy a Roblox Gift Card and give the code to your friend
Use Roblox Premium and create a group
Put a shirt, game pass, or other item on sale in the group or your account
Have your friend buy the item
The Robux from the sale will go to your account after Roblox’s pending period
You can then use those Robux for your own purchases or group payouts
If you want your friend to receive Robux, use a group payout if you have a group with funds
Be careful of scams and never share your password or account details