Don't have an art job, that'd be nice, but for getting jobs in general... you just have to apply. If you can afford to be picky (AKA you don't need the income to live) then do that but if you need the money then don't be ashamed to just apply for whatever you need. Put out tons of online apps and if you have time then walk-ins to places that are hiring are great too. Once you secure a job then you can keep looking for jobs you'd prefer without the pressure of money. Having a good resume is a MUST, if you need have people look it over; for art you need a separate portfolio. I suggest having a few examples but have the BEST examples of the type of work showcasing different techniques. If it's a digital portfolio which it probably will be nowadays please make sure the website is easy to read and navigate.
If you don't think you fully qualify for a job it still doesn't hurt to apply. The worst thing that'll happen is that they won't call back.
Social anxiety is hard to work with, I have some myself but it really depends on how bad it is.. when I first start working any new job it was awful since all the new experiences are a lot to deal with but in time I grew used to it (except my job at the call center but that was just a Bad Time all around for many reasons). If your friend is able to go up and talk to people, even if they're really shy, then they should be fine after a while, though it'll take getting used to. Interviews are awful but you gotta do them; personally I always bring a folder with more copies of my resume (they should already have it by the interview but it's good to have) but its ALSO just for me to hold because having something in my hands calms me down a lot. For art I'd bring a copy of my portfolio on a flash drive or something similar. Have questions prepared for the interviewer, even if it's just "how do I follow up this interview". Unfortunately almost all places will want to do an interview so it's really unavoidable... if your friend doesn't have phone anxiety then a phone interview is an option to ask for.
Ehh I first started working when I was 17, during high school, to start building up my resume and also for some extra money during school.
hmm thats a lot so i'll shut up now, good luck to your friend~