About 2 months ago, me and my friend were charged with possession of marijuana (under), after being stopped for going the wrong way at a back alley in Downtown Toronto. I was the driver of the vehicle. The police searched the vehicle and found about 3g of marijuana inside a closed storage compartment. We did not talk to the police regarding the charges. I was offered diversion program to withdraw all my charges, which I accepted. After my second court date, all charges were withdrawn. My friend is choosing to take the case to trial, hoping to get the charges withdrawn through a "not guilty" verdict. He says it's a better option in terms of conflict with employment or travel in the future? Is it possible for him to subpoena me (co-accused / diverted) as a witness to give testimony?

Yes, he can subpoena you.

If this were in the US, you might try to "take the Fifth", claiming a privilege not to answer questions which would tend to incriminate yourself. It is questionable whether you would be able to refuse to answer, because your case has been diverted.

I believe Canada probably has a privilege not to testify similar to the US Fifth Amendment.

But they cannot use any answer I provide to indict/convict me?

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Originally Posted by isentenceyou

But they cannot use any answer I provide to indict/convict me?

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As AK said you case has been "diverted". This means that all charges were withdrawn pending completion of some sort of service/rehab program. However, that's the equivalent of pleading guilty. So, since you already pleaded guilty and your case was completed, they can't use your testimony in your friend's trial to prosecute you again. This is called double jeopardy and is covered under Section 11(h) of the Canadian Charter of Rights and Freedoms. (Section Eleven of the Canadian Charter of Rights and Freedoms - Wikipedia, the free encyclopedia)
Section 11(h) provides that

“	11. Any person charged with an offence has the right ...(h) if finally acquitted of the offence, not to be tried for it again and, if finally found guilty and punished for the offence, not to be tried or punished for it again;

Now it is possible, that if you provide evidence that would be of a much more serious crime, then the prosecutor could withdraw your diversion. But I would doubt that. Section 13 of the Charter protects you from Self incrimination (Section Thirteen of the Canadian Charter of Rights and Freedoms - Wikipedia, the free encyclopedia). So, if you feel anything you have to say might incriminate you, then you can site Section 13. However, the prosecutor may tell you that since your case as already been completed you may not use Section 13. I would consult a Canadian attorney before you testify to make sure of your rights. And obligations here.