this is illegal, you are
1. changing her check = fraud
2 copying her handwriting = counterfeiting
Also embarrassing people at their work/church is so unprofessional. By all means follow up, maybe a visit to her house will work, but the best solution to your problem is not to take any order without a hefty deposit, and in cases where you go and drop the cake off without seeing the client in person, request them to pay the balance before the delivery.