Just LOOK at the bed during the day to assess hours of sun. It depends not only where the sun is (which your phone could tell you), but where trees, fences, and houses are (which your phone won't tell you). You can get by with just a few hours of sun for cherry tomatoes. But if you don't have a lot of sun, you won't get that much fruit. Leafy vegetables need rather little sun to do very well. Fruiting vegetables tend to need a lot.

Dahlias and lettuce can get by with much less sun than tomatoes.

What she thinks isn't enough could be 4 hours or it might be 8 but if she likes cherry tomatoes gift her a couple of plants and find out. 6 hours is about the break point and even then additional hours of light defused by trees still contributes a lot to plants.

I don't think there is a "break point" for getting fruit. You'll always get fruit. You'll just get a lot more with more sun. True that 6 hours is probably necessary to get the maximum yield. I've grown cherries with 4-5 when I didn't have space at a sunnier spot.

Cherry tomatoes will do much better in part-sun than regular size tomatoes. I grew them successfully in 5-6 hours of sun. I doubt they'd amount to much of anything with less than 4 hours, though. Although, one could always try to see what happens. Time of day matters, too. 4 hours from 11-3 is a whole lot different than 4 hours 7-11 am, for example.