I believe that if this situation was presented to me, I'd ask for the other client's name and ask if I might speak with them... with their permission of course. It could be that the other party had contributing factors that didn't involve the ivy at all, ie: improper drainage, shifting fill soils, inadequate wall design, etc.
Not saying that your man isn't being truthful at all BUT when potentially large amounts of money are involved, I want several opinions in addition to the original one. You might want to get an opinion from a registered soil and/or landscape professional, even if you have to pay for the opinion.
Steve in TX

This post was edited by TXSkeeter on Sat, Oct 12, 13 at 13:25

If the wall is in a good condition there is no problem, ivy is very common over here and i have knowledge of problems of that kind.
Invasive? no doubt about that, thus trim trim trim.