UK visa refusal due to undefined source of fund after retirement

My mother (aged 62) applied for UK visitor visa from Bangladesh recently and got refused. She was intending to meet her elder sister who is a British Citizen living in London. As they haven't met for a long time and my aunt is not in a health condition to visit Bangladesh, my mother wanted to visit her sister on her short holiday at London.

My mother was the sole proprietor of an import oriented company, which she still owns (but doesn't work). But because of her age she doesn't look after the business now. I look after the company on her behalf. I also pay her some money every month. During filling up the UK visa application she mentioned "Retired" in the field where occupation was wanted, which is from her perspective true. But when she filled up "Retired", no further field appeared where she could mention her monthly income (the amount on an average I pay her from the profit of the business, also note that the business is still in her name, not mine). But there was a field that required her to fill up the monthly expenses. So, she filled up the monthly expenses. She had letter of sponsorship from her brother-in-law. She submitted her personal 6-month bank statement too and the amount was sufficient. But her visa got refused where the main reason was shown as:

You state on your visa application form that you are retired. You go on to state that your monthly expenses are 50 GBP. You have provided no evidence of your income or how you support yourself in Bangladesh. Due to this I have been unable to assess your financial circumstances. This also calls into question the origins of the funds you have presented and I am further not satisfied that these funds are genuinely available for your exclusive use.

She is very shocked at this and we are planning to re-apply and submit a new application. Although she is retired, in the sense that she doesn't work anymore, but she still has documents which could be presented to prove her proprietorship of the company.

Here I also provide with the photo of the refusal note:

My question is how should we deal with this problem in the next application. What should she mention in the place of occupation? When we mention she was refused a few days ago, then where and how do we write the explanation?