Should we write 'who he gets along with' or 'that he gets along with'?

The following is a post written by Jennie Bond. What is wrong?

Jennie Bond: I think that William is very close to his father and has accepted that his father is very much in love with Camilla and has been for quite a while. He has taken an adult view towards this and wants to see his dad happy. However, I don't think that he treats Camilla as a kind of surrogate mother or stepmother but as someone that he gets along with. Perhaps as years go by, he will turn to her for advice.
It sounds fine in the original.
Students: Are you brave enough to let our tutors analyse your pronunciation?
We can use both of them. No problem.
Nona The BritIt sounds fine in the original.
Nona, is it possible to use whom in this case?