Hard to say as friends differ. If your friend insists on paying, get them to take you out for a meal (or if you are old enough, a few drinks).
If you are thinking of going into business, say you will do the site but it must link back to your business web site when you set one up. You can update their site later to do this.
First, it gets you to see just how much time you need to develop a web site. Naturally, as you are learning, it will take you much longer than if I was to build it. But when you are confident in what you are doing, you will then know how much time you require to build other web sites.
Secondly, many friends (and customers) think if they have a free or cheap site, they can ask for little tweaks they think takes just a few minutes, but in fact would cost you a few hours or more. If they see what it would have cost in money terms, they should then understand why you have turned down a few 'simple' requests. One of my friends was astounded when I gave her a bill for 2,000 English pounds, minus 2,000 pounds as a waived fee.
I can sell one of my off the peg sites for a few hundred Euros, but a tailored site can cost several thousand.