You can choose to interpret it however you want but it seems like that is what it meant in this agreement or OP wouldn’t be here asking how to change it. If she wanted prepackaged supplements she needed to say that, which shes doing now. I’ve been in several barns (including my current one) where full board DOES mean they’ll feed whatever concoction of supplements the owners want. They typically charge accordingly and have the feed room space to make it work. I think requiring supplements to be pre-portioned is totally reasonable in most cases, but it sounds like OP is mixing full meals including grain in advance which is not common in full board IME. Everywhere I’ve been has scooped the grain fresh and just dumped the portioned supplements on top. I get why it makes sense for OP’s barn setup but it wouldn’t work for a lot of people with harder keepers, especially with limited storage space.
If nothing else I think we can agree this is why a clear, written contract is always a good idea. People interpret things differently based on their backgrounds and having clear expectations in writing protects both sides from feeling taken advantage of.
OP I hope the conversation with your boarder goes well and she doesn’t cause any drama for you!