Sigh.... some of the advice here *face palm*
When IRCC looks at each element of your application, they don't look at just a reference letter per se, they look at the totality of the evidence you provide and how credible it is.
Everyone understands that companies close, people move on and your old bosses never email you back because they hate you (I know because I've tried).
So if you were to put together a bank of evidence in this case that credible SHOWED:
- You and your colleague worked together
- Both your salary slips, contracts and pension payments that prove your company paid you
- Evidence like content, reports, and business cards, for each of you
- Photos where you were badges together at a function
- An explanation of why you're providing alternative documents in lieu of a standard letter
You'll be fine. I ran into exactly your scenario, and I got a letter signed by my colleague who wasn't my manager. We showed photocopies of the magazine we wrote for with our names together, and also her British citizenship certificate and marriage certificate to prove her maiden name and married name, and we linked everything together and how we knew each other, and photos of us at functions together. I wasn't even asked for any additional info because I provided so much detail.
It's the totality of evidence you provide, not just a letter for a letter's sake. Also nobody at IRCC gives 2 shits about your salary. Maybe they'll be jealous if you earn a lot more than them, but that's fine. You don't even have to provide your salary if your contract stipulates that.
Don't bother with a notary. What does that even mean, honestly? All a notary's role is to swear that they saw you sign something, a notary DOES NOT verify the authenticity of the contents of your letter. You must create a credible story. You're welcome.