I am so sorry you are going through this OP and for all those who have suffered in this way ie losing a loved one. I am one too and know how vicious it can be.
Some things that stood out from a previous poster (sorry, I've lost track of the poster - sorry) "maybe it wasn?t God but the devil/evil. If it was why didn?t God step in and help?"
I have often felt the same way, passionately, but I believe he did step in when he sent Jesus to die to break the power of the evil that had come into the world, which polluted everything in it, throwing everything into chaos. The reason he says to believe in Jesus and what he did is not because he is a martinet but because Jesus was (is) God himself who came to live with us as a human being, with all the pain, suffering, joy etc that being human means - he knows what it's like, it's not just theory. Then he willingly died to break the power of evil that has gripped the world, the evidence of which is clear to see. He said, when he had done it, "It is finished", meaning that the job was done.
Imo it's a legal fight though - the other side (whose actions God most often gets the blame for) tries it on. A christian, or a believer in what Jesus did and why, can directly say, no, you can't do that, that's illegal. imo that is what prayer is - or one prayer anyway. That's not to say that those prayers are necessarily successful (for want of a better word): sometimes eg a surgeon with every possible level of expertise can't save a life because there are too many variables, too much has gone wrong on too deep a level. I believe that there are layers of spiritual goings-on ie evil, which can be and often is extremely sophisticated, and which it would take a great amount of eg skill or time to dismantle.
That's also not to say that people who die of eg horrible diseases deserve to, that God couldn't get there in time, that he did it to them, didn't stop it because he doesn't care. 'Death', the effects of death ie chaos, coarses through our world. He has chosen to work through people and tbh a lot of the people who could be praying, with faith, 'your kingdom come, your will be done' aren't ( a lot are busy playing church club, or religion club, or God is horrible and will whack you one if you don't behave club). Mind you, a lot of people who love him do show the world a bit of what he is really like - hence some of us sometimes experiencing a deep love from people who represent him (and, although this is probably controversial, it is my belief that a lot of people are showing God's love without necessarily knowing it eg 'good' people: we all have the capacity to love).
"I?m sorry but I don?t believe that someone/thing/power decided that for a grand plan my dad had to suffer, I just don?t. "
I also don't believe that. If the man who sent this to you believed that then it may be that, like me, he believes that there is just too much he doesn't understand, too much heartache, and has chosen to leave it to a God whom he is convinced is a good God with a profound love for humanity, who understands all the levels of all that is going on in the world. But I don't believe that God actively stands by, like a horrible death is in some grand plan, or says 'tough, sorry, you were determined to go your own way - too bad'. He has given us a choice to love him, accept him, or to not - love does that. That is our choice and he won't force it, will just bomb us with love regardless, if he can get a word in edgeways. Sometimes it is suffering that gives him the 'megaphone' that CS Lewis talks about but imo he would much prefer it wasn't like that, that we didn't have to suffer to experience his goodness and immense kindness. It is as though the glamour of life has to wear thin before we can see him for what he is - a good God, always.