How do I turn my Roblox account into Minecraft?

No, there isn't.
They're two entirely different game which work in completely different ways, and are owned by completely different companies.
There's no convertor.

Your friend is an idiot.

Considering each game is owned by two separate companies with their own marketplace… There's absolutely NO WAY to convert a Roblox account into a Mojang (Minecraft) account & vice versa.

Any site that claims otherwise is definitely a scam trying to steal your account information.