It would help if you told us who will be there [other than the Birthday girl]?
If there are lots of kids, a character dinner might be the answer [if she likes them].
If mostly adults, but some kids, maybe something at Epcot [Le Cellier would be my choice].
If only adults, you could go for Victoria & Albert's or other "up-scale" restaurants.
When you make the reservation, advise them that you're marking her 70th and they may add some special treatment. Actually, it's worth telling the resort you're staying at, all the restaurants you reserve at and get her a birthday button to wear for the duration of her stay. Any number of pleasant treats might come her way and lots of CMs will wish her a happy birthday!
[I've highlighted that it might happen, don't bank on it or even expect it. If it happens, great :sohappy:]
And wish her a happy birthday from all of us here at WDWMagic while you're at it!