Does yahoo have some sort of facility to flag up and help people on the forum who might be at risk?

While there is no 'flag up' such as you've described, we HAVE been assured by staff that if we REPORT the posting as 'illegal' (2nd option) and type in "Suicide" in the text box, staff DOES send information to the person posting regarding services that may help them with their issues. However, they will also remove the posting, as any reference to self-harm or harm to others is considered a violation of the rules of the site. So posting an answer with information may not work, as it would be inaccessible once the question was deleted. Finally, it is good to bear in mind that while there ARE people in distress out there, many of the postings like that actually are being done by trolls. Very sad, but often true.