Firstly, don't panic. If, and it is an if, your mum has lost capacity then while a PoA can't be arranged a Deputyship can, as Saffie said.
But a dementia diagnosis doesn't automatically mean that your mum won't understand what a PoA is, so if the solicitor doesn't know her well then it's worth discussing the capacity issue with your mum's GP or even social worker, if she has one. It's probably worth doing so even if the solicitor has met your mum and formed an opinion. Dementia is often unpredictable and your mum only has to understand what's happening at the time of agreeing to the PoA, not the day before or the day after, and she doesn't need to remember doing so. In short, the bar isn't set all that high.
If it's too late and it has to be a Deputyship then, a bit of form filling aside, it's not that big a deal. It's more closely supervised than a PoA but I, personally, would always want to be completely accountable anyway, so in reality the records I keep for my mum, and my husband's aunt, are as thorough as they would be if no one was having a look once a year.