I've done a combination of full-time jobs and contracts with freelancing on the side, and full-time freelancing, for about twenty years. So I try to keep up with legal and practical ramifications in the USA.
Does your employer prohibit or limit moonlighting? This would normally be in an employee handbook or in a document you signed when you joined the company.
Even if they have such a policy, you should check, because there are labor laws and case law precedents, on both national and state levels, that void some policies like that - even if you've signed that you agree to them.
Don't compete with your employer, or take gigs with companies that compete with them. If your day job is doing websites for a dog food company, doing a website for another pet food company would be competitive, but doing one for a martial arts school wouldn't. And if your employer sells website design services... don't do websites as a side gig.
Don't use your employer's time or resources for your side work. Buy your own computers, books, development tools, etc., and keep them off the company premises. Try to avoid taking phone calls at work, and if you have to, limit them to breaks or lunch, preferably outside your employer's office.
Once you're covered on all those bases, you don't need to say anything to your employer. However, if your boss is reasonable, it might be courteous just to mention, "A friend of mine asked me to do X in my spare time." I do that, and have never gotten grief about it.