I understand your concerns. At 28, I had just come out of a long-term relationship and in no way felt "on the shelf" although I did wonder if I'd ever meet anyone again, or form another lasting relationship. A few years later, after a few brief relationships, I wondered this even more. I was 35 when I met DH, 39 when we got married, 40 when DS1 was born.
In your DD's case, unless she has very real reasons to think she will never have a partner, then I think she's jumping the gun rather a lot, and would hope that the sperm donor/fertility clinics she opts for would counsel her properly (and ideally persuade her to wait a bit longer!)
Single parenting is not an easy choice - especially when there is no possible father on the scene. Plus that's going to be an interesting conversation when the child is old enough to ask about its heritage! (Although again I think the clinics deal with this)
She's not planning on something mad like getting sperm through some dodgy internet company, is she? She would go through proper channels?
Even so, 28 is, IMO, too young to think this way. 35+ is more reasonable.
But having said all that, ALL you can do is voice your concerns, put forward your arguments against, and then tell her that you will support her choice and whatever she feels she has to do. You don't want to alienate her, after all.