I’m not saying she has no right to use it, but it is his name not hers, she was born with a different name, that’s hers.
I read that even Elton John's mum called him Elton rather than Reg - even though their relationship was somewhat patchy whilst she was alive.
As long as it's not for fraudulent purposes, anybody can change their name to pretty much anything for any reason - and that then becomes 100% their own actual name.
Having said that, the cynical part of me does wonder if Angie Bowie, who was married to David for 10 years, had one child with him, who doesn't use the surname Bowie, and who has had two partners since their divorce - the current one for 25 years - would have kept that surname had David been David Smith the postman from Slough.
However, she has every right to do so - she didn't only 'borrow' it from him on their marriage, and she might well say that her own career and public profile was at its height when she happened to have that surname, so that's her professional surname as well as it also having been his. After all, he originally took the name from Jim Bowie and was never even married to him!