I agree with JollyGG and LaughingHyena here - you're both being reasonable, which means the best thing to do is probably to skip this party.
Plus, you just started this this past Monday? So, eight days ago? I'm glad it's going well, but in my experience, it can go any direction from here. Potty training is not necessarily a linear or predictable process, and a kid who just quit Pullups last week is not low risk for accidents when distracted by a party and (probably) hopped on sugar.
I can see why your friend would prefer to limit the risk of potty accidents in a crowded bounce house. That's really not a crazy thing, even if she's not responsible for cleaning it out. Adding liquids to that situation is an additional risk for injury, and it's an unpleasant early end to the birthday if half a dozen kids come out of that thing with pee smeared all over.
It's no skin off your back, really, to admit that potty training can be an awkward stage in a kid's social life, and plan to skip some parties while you get that nailed down really solidly.