- He will be of great help to you.
1a. He will be of great help for you. - It is a great help to you.
- It is a great help for you.
Are these sentences OK?
- He will be of great help to you. – OK.
1a. He will be of great help for you. – Not impossible I suppose, but doesn’t seem very natural. - It is a great help to you. – OK.
- It is a great help for you. – OK, but “to” is probably more usual.