I think you have to respect her wishes, it may well be an ethical thing, and she may feel obligated to you as pointed out before, and it seems it makes her uncomfortable, i think some of the ideas here are great and would be better appreciated than a cash gift,or maybe you could offer to donate some time, in helping her set up her practice, a nice bunch of flowers on opening day and a nice card from the furbabies, i think the best gift you can give her , is to pass on to all your friends and family just what a great vet she is.