I would suggest for someone with previous exposure to web development and some design flair: learn Javascript. There are great boot-camps available and if front-end UX/design isn't in their interests they can fall back to Node.js
An alternative might be Python. It's an easier language to learn than Javascript since there are fewer "features" one must learn to avoid. It's also rather prevalent in the web development space. And the bonus is that if, later on, they want to transition to another area of expertise there are fields such as scientific and cloud computing that use Python rather extensively.
If the other friend is fluent in DSP and has some exposure to C I would suggest staying on that tack... take a refresher course in C and possibly pick up a scripting language like Python on the side. Check out Art & Logic: they hire remote developers and work with many clients on DSP-related projects.
An alternative might be Python. It's an easier language to learn than Javascript since there are fewer "features" one must learn to avoid. It's also rather prevalent in the web development space. And the bonus is that if, later on, they want to transition to another area of expertise there are fields such as scientific and cloud computing that use Python rather extensively.
If the other friend is fluent in DSP and has some exposure to C I would suggest staying on that tack... take a refresher course in C and possibly pick up a scripting language like Python on the side. Check out Art & Logic: they hire remote developers and work with many clients on DSP-related projects.