I kind of agree with LW that this is not really an HR situation. Like, yes, it's embarrassing to ask the person directly to be more discreet. But why should an HR person have to be the one to deal with the embarrassing conversation? Also I think what the guy is doing is somewhat poor manners, but it's not that awful, I don't feel like he deserves a formal reprimand or other serious career consequences. If he's open to a friendly request that should be the end of it; the scenario described doesn't sound like deliberate sexual harassment to me.

If this had been a Miss Manners letter instead of an AAM letter, you'd probably get a suggestion of just casually happening to mention that it's annoying how the stall partitions are really thin and you can hear much more than you want to. That's probably enough, assuming the coworker isn't deliberately being loud for kicks.