I don't see any overcurrent devices on the splitter you linked. Just sayin'.
BTW, I had not originally fully read your post. The fact that there's only one outlet on the circuit is fine. I'm not a big fan of the splitter, but you do you. Its >probably< not gonna burn your house down while you sleep, and that's good enough for me.
I have that splitter, and you are correct it doesnt have an overcurrent device.
But assuming his 14-50 circuit is to code for EV charging, and has a GFCI 50A breaker, he really shouldn't need an additional overcurrent device in line.
I use the splitter at a vacation spot that has a 50A circuit for my Travel Trailer. The only time I'm not charging is if/when I need the AC on in the trailer. But I also have a 15A outlet right next to the power post so I can trickle charge on hot days.
If he is only ever going to use 15A of the 14-50, he should really just get a long extension cord and use the free standard outlet adapter that comes with the car.