Instead of going through legal channels for POA could you convince your mum to put the money into a bank account which require both of you to sign for withdrawls? That way the money would be safe until she finds a house or decides how to invest it.

I do think the first thing to do is to sit down and tell her your concerns. If you phrase it gently, she shouldn't be offended and she might (might!) surprise you with her response!

You mentioned your dad, but I'm assuming they're no longer together?