Actually, rereading the OP, I see she said "threadbare clothes." So I guess that means the clothes are really worn out. Sorry, nansoval, that I didn't pick that up before.
Still, I don't know if there's much you can do. If she's of sound mind, there might not be. She's an adult and has a right to make her own decisions.
Gifts are probably the best solution, IMO, if you know her sizes, and so forth. There's no guarantee she'll wear the new clothed, but at least, she'll have them, in case you're worried about social services, as Wilma suggests.
Was she always like this about clothes? Maybe she grew up poor and is used to being exceedingly frugal? Even though she can afford new clothes, if she has a "poor person's mentality," she may still be very cautious about spending.
Or is this a recent thing? If her clothes are so shabby, how is she in other areas of caring for herself? Is it time, maybe, for some extra help, like a hired carer?
One other thought... if this attitude is fairly new... could she have become depressed? Has she had an overall checkup, lately? You probably can't force her to seek help, anymore than you can get her to buy new clothes, I know. It's just a thought.