Hi Ace,
I think your friend steered you right. I haven't had any back or any serious skeletal/joint problems since beginning Wing Chun. The practice seems to strengthen the body while doing minimal stress damage. I don't know of any art which is less likely to harm your back, but the risk is still there.
There are differences in Wing Chun lineages as to the degree of twisting incorporated into their forms, chi sao, and other expressions of Wing Chun. Yip Man Wing Chun emphasizes low kicks and straight up the middle tactics while holding the trunk aligned. This last should be supportive of building up lumbar strength while minimizing torsion of the vertebral column.
I don't know where you're located, but if it's the Bay Area, check out Ken Chung or Ben Der of Leung Sheung lineage or WT, Leung Ting's or Emin Boeztepe's lineages. Or if you're elswhere, Chu Shun Tin, Lok Yiu, Lo Man Kam, or Augustine Fong lines. All of these are the traditional Yip Man focus on structure. I don't mean to exclude any other Wing Chun options, but these are the lines with which I am most familiar. Of course, I like my own (Ben Der) the best.
Obviously, you want to explain your needs to any potential teacher and find out what he or she thinks about their style in relation to your problem. I'd avoid any school which emphasizes high or non-Wing Chun-style kicks for obvious reasons.
Good luck. Be well.
John Weiland
"Et si fellitur de genu pugnat"
(And if he falls, he fights on his knees)
---Motto of the Roman Legionary
"Aim at Heaven and you will get earth 'thrown in': aim at earth
and you will get neither." --C. S. Lewis