1 - decide on a rate
The rate should be based on your experience, your living expenses and what kind of work your doing. If someone wanted just HTML / CSS / JS then I would charge differently then someone who wanted a full custom Frontend and Backend.
2 - Get a contract!
Before you do anything else - watch this - Now - go get a contract.
3 - Make a quote
Figure out how long the work will take you - times your rate times 2 for crap happens - that's your rate for this project. If you think it's to high - you can round down -, but if you think it's to high, why is your rate what it is? if you think it's to low - adjust your rate. But try to keep the rate consistent across projects.
This sounds like your first project so it's going to be hard to come up with a rate at first. If your doing this just because it happened to fall into your lap, that's 1 thing.
If your going to try to consistently use it as a side gig, then you need to take the rate very seriously. A rate should be based on what you need to live off of. Rent / Mortgage expenses. Car loan / lease expenses. Food / Entertainment, etc... Medical, Dental, Vision insurance. How much does it cost you to live. Once you figure that out, you can figure out your rate. It's different for everyone.
And no one wants "a simple app with a few views" - get exactly what they want down on paper, put it in your contract - make it clear as water that that's what your building, and get both sides to sign it.