NTA I feel uniquely qualified to comment on this so I'm going to admit for all the internet strangers that read this that I have the exact same issue haha. No matter how many times my partner tells me he doesn't care about possibly overhearing that stuff, I still feel self conscious about it. I used to run the hot shower but realised that doing so is completely untenable as it adds up over time cost wise.

An alternative: (while not ideal, is a compromise) run a COLD TAP so it's less water and doesn't run up the bill. Another alternative: COUGH LOUDLY at the right moment and it covers it just fine, or even put a small amount of tissue in the toilet first to alleviate any splashing sounds (gross, TMI, ew). I'm not sure how comfortable you'd be suggesting alternatives to her, but these are all good ones. Loud music plus hot shower is really unnecessary, and that is coming from someone who has the same issue.