Visitor visa refusal

My mother applied for a family visitor's visa in Feburary to visit me (her daughter) in the UK. However, after 2 months she received notification that her visa was rejected due to a lack of information about her last exit from the UK more than 15 years ago. I would like to know, how can we prove when she exited the UK since she does not have her previous passport?
There should be some way of finding this information otherwise how would she ever be able to come visit me here?

Just to let you know I'm from Iran and currently doing my PhD in the UK.

Thank you in advance for any information you can provide me.

Sincerely,
Samaneh Soleimani

Hi Samaneh,

My parents are also from Iran and went to Istanbul afew weeks ago for their visit visa interview. Unfortunately they received their refusal through post 3 days ago and there's no appeal letter with it. we would like to appeal against this decision and ask for a reconsideration but don't know how and where we can post the requested documents. thought you'd know by now as you went through the same process.

thanks,
anna