Because all cats have different personalities, likes and dislikes, some methods may work well with some cats and not at all with others. It's a case of trial and error.
1. If you have bare soil cover it with gravel, pinecones or slate chippings etc, it will discourage digging.
2. Gather up many pine cones and place them amongst the flowers.
3. Cats hate the smell of citrus. Get yourself some oranges and scatter the peels and segments too if you want around in your garden. You can also spritz the area with orange or lemon scented water
4. Try coffee grounds.
5. DON'T use cayenne pepper - cats get it on their paws, then in their eyes and scratch their eyes -painful and cruel.
6. Plastic forks (big pkg for $1 or so), but bamboo skewers would look much better - stick them in the ground all around plants and flowers!
7. Try crushing eggshells and sprinkling them around the flower bed. The prickly effect of the shells on the cat's paws may deter it for a while. Best to do this when you are expecting a few days of nice weather - rain will just wash it into the soil. Another benefit - it is also reputed to deter slugs.
8. Spray water at them (not from a hose) from one of those spray bottles from a garden centre or use a super-soaking water gun for cats further away.
Start with the cheapest method. You don't need to spend a lot of money to deter cats.
And talk to the cat owner about the problem and let them know what steps you are taking. Make sure they know you don't wish to harm the cat, just that you're trying to protect your flowers. They may have some suggestions that would help - after all, they know their cat's personality better than anyone.