My advice to the author would be to take the website down, change the domain,repostory binary name. There is no realistic scenario where this (as it is) survives thismonth.
So it shouldn't be "Whatsapp" for something. But "Something" for Whatsapp. I'm not entirely sure if that avoids all the legal issues or whether it was just something Reddit itself was willing to compromise on in the case of "Relay for Reddit", but at least you should start with that kind of default rather than shamelessly using other company's product name as if it was your own.
I think others who have used Google's name or other product names in their own Chrome extensions have taken a similar path. Like how "Gmail Checker Plus" (or something) became "Checker Plus For Gmail" [2].
[1] - https://play.google.com/store/apps/details?id=free.reddit.ne...
Much older examples include "Plume for Twitter" and "FriendCaster for Facebook". In the case of the former, it was by agreement with Twitter: Plume used to be called Touiteur until Twitter made them change it.
You're seeing it more lately because Google recently changed the Play Store's app naming policy to forbid using the name of an existing service as part of your app's main brand, only allowing a special clause (e.g. "for [Service]") following the main brand. Also, around the same time, reddit changed their API policy to add a similar requirement.
In the long run, it's going to be beneficial for app developers because now they can take their name and use it for apps across multiple services. For example, the developers of Plume could create a "Plume for Facebook" app and so on, so they end up with a whole family of Plume apps, each one for a different social network.
And for good reason. Remember that trademarks are a consumer protection policy to avoid confusion. I was certainly confused at first. If it wasn't for the "Show HN" on the title, I might still be confused. The product name is "WhatsApp for Desktop", the page includes links back to whatsapp.com for the user to find more information, pictures copied directly from whatsapp.com (itself a fairly obvious copyright violation), and the file that's downloaded is called WhatsAppSetup.exe.
It is quite reasonable to think that someone could think this was official, easily missing the fine print that clarifies.
I strongly suggest anyone releasing software on the internet familiarize themselves with the relevant laws on trademark and copyright so they can avoid mistakes like this. There is no way this could stand up to legal scrutiny. Naming your product AnotherCompany PrepositionalPhrase is not something which presents an interesting legal debate, it's an amateur mistake.
I am not a lawyer. This comment is not legal advice.