It’s a serious health and safety risk.
My DP is Asian and when we first started seeing each other his cousin visited my house and tried to do this on my toilet seat.
My toilet seat was loose and broke completely with the weight, he went flying forward and knocked himself out, that was a fun afternoon.
Is it possible you can talk to this lady one to one and say you are aware this is happening and you are worried about her safety. Be matter of fact so it’s not embarrassing and don’t skirt around things. Ask what you can do to make her feel more comfortable but make sure she knows you will be discreet and the conversation will go no further.
You could offer a squatty potty as mentioned above, is there anywhere private to keep it in the toilets? (I’m not familiar with them as DP uses the toilet as intended!)
Is she washing afterwards? You can get a portable bum washer off TEMU or Amazon, we have a bidet attachment at home that’s discreet and fitted into the toilet seat.
I know it’s an awkward conversation but it’ll be more awkward if she hurts herself, every time she stands on that toilet seat it’ll be loosening and she might be adept at keeping balance but not if the seat moves suddenly.
Don’t put up a sign, she’s going to know it’s directed at her and would you want it bringing to everyone’s attention? It sounds like you are genuinely looking for a diplomatic solution and to be discreet which will be appreciated.
DP’s cousin struggled to use western toilets, if you went to another country with unfamiliar toilets you’d be the same. DP takes the attitude he lives here so he follows our customs but he appreciated the bidet so much because it’s what he’s used too. I couldn’t be without it now, it’s so much more hygienic!
Don’t risk a serious injury over a possibly awkward conversation, the conversation will be over quickly but if this lady hurts herself the implications will be far longer lasting.
As for people commenting about the hygiene aspect, it’s not pleasant to sit on a toilet that’s been stood on, if there is no other solution can you at least get seat covers? If the aim isn’t accurate it can be messy as well.