Yes, PEY folks do try to (and usually quite successfully) use the J toilet. It is an identical situation on 33E/33K and 77G. Adding to the fun, I have no fewer than 5 situations on 77G and 33K/33E where people self upgrade themselves. I have to imagine it will happen on 35G occasionally too.

Definitely tell the FAs in either situation. They both drive me nuts, particularly since the rear mini cabins are in theory lovely.

The WORST for the bathroom dynamic by far is 77G, because the toilet is actually inside the cabin (versus the galley) on the A side aisle. PEY pax poking their head through the curtains see it and some flights, it turns into a constant stream of people, even to the point where they're lined up in the J class A side aisle next to the seats. Lovely. And because the extra galley curtain is usually closed, the FAs don't see it.

35G/33K/33E are a lot better, since the rear J lavatory is inside the galley area, so it's a bigger stretch for the pax to cross the cabin. Often they don't know where the lav is, walk forward and then get swatted by the FAs. And them getting sent back deters others. Fyi, I have selected the rear mini cabin for 3 out of probably 7-10 A350 flights to date. All short haul, but lovely every single time. I also frequently fly in the mini-cabin on 33E and 33K, which is an identical dynamic. But I refuse to fly in the mini-cabin (particularly left side) of 77G.

I don't know the service flows and staff stations on A350, but overall the rear mini-cabin - because it is directly connected without a galley to PEY, and the rear J galley is very close, just two rows ahead (giving possible cabin crossers the view that there is just two rows of J to cross before the reward of a potential bathroom nearby) encourages cross cabin traffic. This is just normal human behavior. And when the FAs aren't patrolling, which is natural when you have a cabin dynamic like that, then it will happen more and more if a few folks are successful.

Interestingly on 77H, cabin crossing rarely happens. Even though row 26 of J is connected to PEY without a galley. I've concluded there are two reasons for this. 1) PEY pax have their own lav. 2) because the rear J cabin is enormous (11 rows), and if they're peeking through the curtains PEY pax see they'd have to walk through the entire J cabin to get to a galley, all 11 rows of it. It's way easier on the 77G/35G/33K/33E because it's TWO or three rows to cross only. Not 11. On 77H, note the left PeY section doesn't have a lav, but I frequently sit in 26A (last biz seat) on longhaul flights and very, very rarely have pax crossing the curtain. So that's one reason I say I think a bigger J class deters crossers.


Whereas on 77G it's a virtual guarantee to happen many times each flight. And of course, once some pax do it they all do.

Anyway long story short tell the FAs don't hesitate at all.