MIL giving toddler pop

My LO is a little over a year old, and we usually don’t have any issues with what she feeds him. I don’t mind the junk food here and there, but I’m starting to get worried about her giving him pop. I’ve seen her give him a few sips of watered down pop here and there, but today she came to my house and gave him probably 5 sips of straight Diet Mountain Dew. I told her no more, and she didn’t give him any more and then left. She came back later in the day (she borrowed something and brought it back), and took the straw out of his water glass, stuck it in her can of diet Mountain Dew and let him drink it. I said “Okay that’s enough, you had some earlier.” She said that it was fine because he’s napped already. I told her that wasn’t my reasoning, it’s just not good for him. I’m worried now that she does this at her house, and he’s there 4 days a week for 5 hours. (She’s our only source of childcare) Should I talk to her about it, or let it go? I never wanted him to have pop, but let it go because it wasn’t much and was watered down. But doing it at my house bothers me because I want him to want water, not pop and juice. After she did that, he kept taking his straw out and trying to put it back in her can and wouldn’t drink his water 😣 Suggestions?