hi
@Summereve
a warm welcome to DTP
you say your mum gets ready, but does she actually leave ... in that, if she's calm getting herself ready and doesn't actually go out of the house, then maybe just let her be; it's a bit of a nuisance but no harm is being done ... after a while, make her a cuppa and see if she will sit and chat or watch TV, if not leave her to herself as long as she's safe
if she tries to leave, maybe just before that suggest you sit and have a cuppa and a treat while you make a list of shopping, work out a route, say you'll just check the TV as her favourite programme is due and you wouldn't want to miss it ie any kind of distraction she will accept but not telling her outright you won't be going out