If you have an EPA and it has been registered because they now lack capacity, changing their will is out of the question unless the Court of Protection get involved and make a statutory will.
If they still have capacity it is, of course, possible for them to be bullied into changing their will and the fact that you had not been informed is neither here nor there. Challenging a will on the grounds of undue influence is extremely difficult since undue influence has to be proved. However, challenging it on the basis of lack of knowledge and approval is somewhat easier. That said, prevention is better than cure. Try talking to this person and explain your concerns. I would also suggest that you keep a contemporaneous note of everything that happens - it is easier to recall details from notes rather than from memory 10 years later.