xiao bai tu, please advise

I am in the process to submit 140. and
I was asked by my lawyer to provide the employment verification letter (reference letter) from my previous supervisor. The purpose of the letter is to confirm the immigration officer that I have the required working experience.

I have left that company long time ago, and the company has closed. so I could not find him. But I did find a colleague and she has signed the letter from me.

My question is that is my colleague's letter okay, or I definitely have to find my previous boss? or will it help to find more colleagues to prove my working experience.