Thank you very much Deejaus you are spot on judging by images I have been able to google.

Horse chestnut Aesculus hippocastanum. But I wouldn't describe the flowers as yellow. Creamy white with pink stamens. Here is a link that might be useful:

Take a look at Sida acuta or S rhombifolia and see what you think.

Thanks, everyone. The family does grow all sorts of fruits and vegetables back there, many vining. They've built structures and trellises to support, so this rogue vine took me a bit by surprise. After last year's Passiflora incarnata debacle -- I don't think they realized what they'd planted, as it took over their (and my) yard twice, one time completely engulfing their apple tree -- I'm a bit leery when I spot something new that might end up in my yard.