Can I scan data from a paper for my own research?
It might be helpful to consult some legal assistant at your university, because there is a bad relationship between our group and their group, so if they are really determined, they might want to make problems, so it would be good to have a backup plan, or just be sure that you are really not breaking any laws, and that your party will stand by your side in case things from the other group's side become dirty. (This is an awful situation, though.)

But that aside, the line of reasoning is quite straightforward: anything one publishes becomes publicly available and viable for research by others. I understand that they didn't publish a table with numerical data, but if you think that recreating it from a plot is methodologically fine, then in your paper you should just describe the method and of course give credit to the source.