my GFs Ex BF with cancer

I got to know a very worrying news about my GF.
We have been in a committed relationship for 2 years now. We spent quality times together and texted or called when we can't. I had no doubt that she was seen only me. Coz of the time and effort she invest on me. Also we had miner issues.

In the beginning, She told me about her ex, who has eye cancer which was the reason for there relationship to end.

One of the main issues was, when ever I asked about where she lived and if I can come over, she went totally mad and said I don't have to come there ever. She gave me various reasons why I should not visit her place. Those reasons didn't convince me. So I did some investigation found her place. Then i went there to suprise her but she didn't take me in to the apartment . It made me very mad, so we started having issues. Then after two days she invited me to her place to stay over. I was happy .

But after I left she told me that there was a reason for all this. She said she wanted to help her ex coz there is no one to help him. So he has been giving him meals to take away when ever she can . She spoke to Him every week. But she said she did just those behind my back nothing sexual happend. She said she did it purely to help her ex suffering from cancer.

My question : obviously she had lied to me for an year. But she is sayin that she will give up helping ex and totally disconnect from her ex. She is begging for another chance coz she loves me so much.

So now that you found out about it all of a sudden her ex doesn't need her help anymore? This woman has lied to you for an year, I think you should dump her.

How do you know he even has cancer maybe she lied about that too?

Thanks for your feedback.

If I were to give her another chance, how should I approach it. What should I tell her to do. How should I build my trust? Is it even possible.
This is assuming that she has been honest with me, and has told the truth now.

If she has told you the truth now, it means that her ex is seriously ill and badly needs her help, since for the whole past year she has been helping him because he had no one else. So now she's just going to stop helping him? A seriously ill person in need of help, alone in the world?

yes, but he is not seriously ill. He is going to loose his eye sight after another 6, 7 years. He is working full time at the moment, living a normal life. He is an immigrant just like and me and my GF. He has his parents and relations in his home land. So there are other people.

It is really hard problem to deal with. I dont mind helping anyone in need, but lying, mistreating/blaming me for that course is the worrying part. I am not sure the things shes now saying is true or not, what happened is exactly that or not.
She is going to help me to build my trust, but I am not sure how I proceed, what things I should ask from her.
How do I investigate what she has told me is true or not.