Didn't include one AS grade

I know a friend (it isn't me haha I'm in year 11) that applied to some universities for Engineering but didn't submit a D grade in the UCAS form that she got for her AS but then moved to a different 6th form and redid AS with different A-levels (so she didn't carry that on).

What would happen to her? would the universities find out?

She got a D, but then retook it and got a higher grade (and this is the grade that she declared)? Or did she not retake it?

no she did not retake it, but she isn't declaring it.

She needs to declare it, otherwise the university could cancel the offer or kick her out when she's there. It would be much better for her to contact UCAS and the universities now and say "Oh, sorry, I forgot to include my D at AS whatever". They might cancel the offer, but better now than results day or a week from graduating.