A letter of invitation can help your application, but it's no guarantee you'll get it. Your friend should write it, notarize it and send it to you to submit with your application. You'll find everything about it here.

The CIC website is vague about when you need one, it seems the visa office may ask you for it "sometimes".

Sometimes, when you apply for a visa to visit Canada, we ask you to give us a letter of invitation from someone in Canada.

A letter of invitation does not guarantee that we will issue a visa. Visa officers assess you to decide whether you meet the terms of Canada's immigration law.

But you can also send one in yourself with the application.

You may provide your family member or friend a letter of invitation in support of their visa application. [...] A letter of invitation can help, but it does not guarantee the person will get a visa.

A letter of invitation can help your application, (it's no guarantee though) and is not discouraged by CIC, there's no harm supplying one. Your friend should write it, notarize it and send it to you to submit with your application. You'll find everything about it here.

The CIC website is vague about when you need one, it seems the visa office may ask you for it "sometimes".

Sometimes, when you apply for a visa to visit Canada, we ask you to give us a letter of invitation from someone in Canada.

A letter of invitation does not guarantee that we will issue a visa. Visa officers assess you to decide whether you meet the terms of Canada's immigration law.

But you can also send one in yourself with the application.

You may provide your family member or friend a letter of invitation in support of their visa application. [...] A letter of invitation can help, but it does not guarantee the person will get a visa.