I am assuming that the original owner wants to have you give the go ahead to breed is because he has a mini Hereford bull. A mini bull won't be able to breed full size heifers and cows with great success on a large scale basis. He may be able to stick a few shorter animals, but not a big Holstein heifer.
I would hold off and breed between 18 and 24 months for a dairy animal. She will be a bit taller and her bag won't drag in the mud. If the 1\4 that you don't know is jersey, she may be a smaller cow anyways. If it's something like brown swiss or milking short horn, she may end up on the large side.