Hi Martin,
Great question. I run into this situation all the time just like trying to tell a potential customer that their Flash website has to be redone.
I usually call them and try to educate them on what Google is now requiring. That poor link building practices work in the past but not today and that their a in danger of getting dinged by the Google Spam team. I also try and fine one good article that explains all this from a very reliable source so that they don't think you are making it up.
Education is the only way. I find educating my clients the best way to sell white hat linkbuilding services. Yes it is time consuming but effective and it builds trust.
I would not bash the other SEO company. I would just explain that Google has improved their algorithm and that these techniques are considered black hat or poor link building techniques and that they should correct them or else....
Hope that answers your question. The same issue pops up with websites that have poor web architectures. When you suggest to the client, that they need to modify their website, they freak out.
Good luck and I hope this answers your question
Carla