Discretionary leave for an EU citizen

[Bubulin]

Hello,
One of my friends is an EU national and holds passport of his country. 2 years ago he was trafficked into the UK by some criminals and exploited until the police rescued him. In February 2018, he got limited leave to remain for one year as a victim of modern slavery, on the basis that he is assisting the police with their investigation. My friend holds a biometric residence permit and has a letter from the Home Office, stating that it is discretionary leave which doesn't mean he would hundred per cent allowed to live in the UK in the future...
I searched on Google and found out that discretionary leave is often given to those rescued from slavery, but why is it often needed in cases of EU nationals who have automatic right to live here? If the police want to contact my friend and ask for his help, they could do it without any extra documents I guess...
And my main question is: what is going to happen to my friend after his permit expires? In the letter it is said that he should either reapply or depart the UK. But again, he is an EU national, working, has accomodation, almost recovered from his experiences... The police have recently informed him that the case with those criminals is now closed, they are punished, so probably they won't need him anymore.
So will he have to go or be able to just keep exercising his free movement rights?

[Casa]

Why would your friend need to be smuggled into the UK if he is an EU national ?

[Bubulin]

He was promised a good job when being at his country, came here with very bad English skills and no money, and was being exploited.
The law says that such people are often unable to exercise their free movement rights due to poor language and lack of money, even if they are EU citizens.
So he received help that he needed, but what is going to be later we don't know...

[Casa]

Why would your friend need to be smuggled into the UK if he is an EU national ?

I still stand by my comment above. What EU nationality does he hold