My mums friend lost her brother a few months ago, and she went to a spritualist/medium about it.

The medium didnt know her at all, but told her her brother was happy and reunited with his ex wife who died a few years before him.

Amazingly, the man did have an ex wife who died before him. The medium didnt know anything about them but somehow gathered this info through communication

That is how Mediums work. The soul, now in the afterlife, generally wants/ needs to let their loved ones know all is well with them. This helps them to 'let go' and adjust to their new surroundings.
They try, with varying success, to impart their intended communication to the Medium, who, again with varying success, receives all or some of it.