reoccurring mold can be difficult to remove since you need to kill the spores. Bleach and chlorine will kill the visible mold but if the spores are not killed it will keep coming back. There are special treatments such as Ozone and others that can work. I would clean it real well with a bleach solution, keep it dry and tell them they may need to call a mold remediation company. There are some chemicals you can buy that claim to kill the spores but the following tips will also help
1. Have the customer squeegee the shower walls each time they shower
2. Try to keep air circulating in the shower. Leave the door open after a shower
3. Change the air condition filters monthly to keep mold spores low in the air
4. Have them clean the shower on a regular bases.
5. From time to time place a fan in the shower.