I'm so sorry to hear your Dad's news but am hopeful that his oncologist will have a treatment plan ready to discuss on Thursday. I've learned that incurable doesn't mean untreatable.

I think the advice to attend alone will be due to COVID and I suggest your Dad ignores it. Through my treatment this past 6 months I have attended alone when it was for things like a scan or pre-op checks, but when I knew I would be receiving a diagnosis or results I took my husband with me - and made him take notes! He's always been welcomed despite what the letter says. I think it's really important that your Mum is there too - to offer support, to be a second pair of ears and to ask her own questions if she would like. I am sure she will be welcome but you could always call up in advance to confirm that she will be coming and, if you do have to have a slightly awkward conversation, then at least it's happened in advance.

Sending very best wishes to all of you.

Thank you for the reply that was really kind and you have set my mind at ease. 
It is so easy to get carried away by even the slightest thing at the moment and my mind was racing.

I hope you and your family are coping ok through your difficult time. 

Kind Regards