MIL please help!

This is my first week back to work. I work 45-55 hours a week. Our three boys are with mil when our work schedules overlap. Usually three- five days per week. We pay her as well. I've had some issues this week with her not understanding I have breast milk in the fridge and which to use etc verse the formula I have for just in case. I don't speak Spanish, she doesn't speak English, so it has been a struggle. I thought we had it ironed out, but my SO brought everyone to my job for lunch today, and in walks my mil feeding my 7 week old a bottle of water! Now, I don't think it's going to hurt him to have water, but it's not something I am familiar with. I asked my SO why the baby has water and his response is that's "their" way and if I have a problem with it to talk to her myself because he doesn't want to get in the middle. I don't want her to think I don't trust her with my kids or be the crazy daughter in law, but I am starting to get really frustrated. Am I overreacting? It's not going to harm my son is it? Because I am afraid insinuating she doesn't know how to care for a child (she's had five and cared for too many grandkids to count) will certainly harm our relationship....