So I tried your web app on my phone (which means using the URL you provided on Chrome on mobile / Firefox on mobile), I selected, copied and pasted text from another website into the textarea and pressed enter, it ran all the translations for the text, all went smoothly it works like a charm for me 
BTW lovinā the app !
So I donāt really know if he had another problem with its phone, or the web connection went awry, or if I correctly understood the problem, but using your app on a browser on mobile works well on my side. You could try and ask for more details, or if possible send him a demo video of how it works on your mobile.