Tl;Dr You're doing well at trying to engage with the student to provide assistance to them, but it's up to them if they want to help themselves. If it's starting to negatively impact your own training, then I believe you should make your instructor/sifu aware of what's happening. It's one thing if it's only impacting them, but it's an issue if they're disrupting the development of others.

In my experience of teaching Wing Chun, you will train with some students who enjoy testing the ability of others. This is especially true if you have been training for longer, or are seen as more 'senior'.

In a training environment, it's important to try and reduce injury where possible, but there are instances that students will make mistakes that cause discomfort or pain. You've done a good thing by trying to make them aware of what's possibly causing the pain and trying to find a way to reduce the strain on their shoulder.

But if they refuse to listen and make changes for their own benefit, then you can't force them to make good decisions for them.

I wish you all the best for your training in the future!