Oh your poor Mum I am sorry that her friends haven't been stepping up like you would normally expect friends to and I'm very sorry for your Mum's and your loss💐.
I don't know if it's something available where your DMum lives (or if it's something she'd enjoy) but has she thought about joining a group, there's tons where I live that offer allsorts of hobbies,day trips,meetings,it ranges from shared allotments,Tea and dance afternoons and evenings,exercise groups,swimming groups(some at Leisure centres,some wild swimming)Bingo trips,day trips to the coast,to National trust places,museums,visits to city's,cookery classes, photography groups,days at the races,chess club,computer classes,art lessons,pottery classes.
They honestly cover lots of different interest's (sorry I can't remember them all)and the days out trips always include travel there and back so are ideal if you can't drive or don't own a car.
Most of the ones local to us are run from local community centers and are run by alot of volunteers and some paid staff from the local council,some are run by different local churches,others are run by our local council and some are run by local residents groups.
I hope something there helps.🤞