Securing a 'Permanent' Tenancy

Hi grateful for some advice on this :-

My uncle is in his 70's, recently met a woman in her 40's. The woman has walked away from a 20 year marriage with nothing - saying she didn't want anything although joint estate is supposed to be around a million.

She has persuaded my uncle to sign over half of his house to her because she says it is her belief my uncle's daughter will evict her when he dies.

My uncle is estranged from his daughter, I'm not particularly worried about her, but she has an autistic son who is really going to need financial help. It has always been my uncles intention to leave everything to him. Am I right in thinking if he puts her on as joint owner, when he dies she will have full control of his remaining 50%. She is coming round to me as she wants me to be a witness on the TR1. I don't want to sign it because I am of the belief there are other ways of securing the property for his girlfriend after he dies, but basically keeping the property safe for his 5 year old grandson.

I'm really uncomfortable with this; can anyone offer a solution please?