This is assuming you want the following outcomes.
1. Compensation for unpaid medical bills only.
2. Changes in corporate practices for food allergic customers that are reasonable and not ending only in a refusal of service to all dairy allergic customers and/or bigger CYA signs.
- Have DD write timeline exactly as she remembers like a witness statement. Refer only to that statement. Immediately stop all usage of "I know" "I feel", etc. Stick to times, statements, actions.
- Appeal to corporate interests that are shared interests with customers. Fact: food allergies in some markets are approaching epidemic levels. Fact: they will not be able to avoid serving all allergic customers
Use Sloane Miller's approach by showing how adapting practices to the reality of a growing customer base allergic to a variety of foods will help them as they help customers. Request they adopt practices to reduce contamination including employee training, separation, color coding, and the like.
Show in a timeline by DD that she met her burden of caution, that she more than reasonably sought clarification, that customers like her have been able to continue patronage but for errors that could reasonably be reduced with small changes in practice such as color coding. In the meantime because the error was made without any mediating practices you request unpaid medical bills resulting from the wrong drink be paid plus a request to put into place sensible, effective practices on the corporate side to increase service to the market that will continue to grow with allergic individuals.
The one thing I would not do is allow myself to be maneuvered into a position where the claim essentially becomes
Company established with a dairy based menu, you owe me because we were served dairy.