Ummm. Your nutritionist/breeder friend... are they a veterinarian? Do they have a license in your state which allows them to diagnose diseases and prescribe medications and treatment? If not, then they are 'practicing veterinary medicine without a license', and are likely in violation of your state's Veterinary Practice Act.
I'm not trying to be an @ss, but ...there are reasons that most states limit who can practice veterinary medicine.
I spent 9 years in college and veterinary school,then another 4 years in grad school + 30 years of experience... in order to be entrusted with that responsibility. I try not to raise a stink here too often, but I see a lot of 'recommendations' from folks who may or may not know what they're talking about... sometimes totally inappropriate... and that's usually when I weigh in.
Do this person know the mode of action or indications for prescribing dexamethasone?
Is this cow bred back? I would hope so, since she's got a 'nice 300 lb heifer' on her. Does your breeder friend know? - or have they advised you - that administering dexamethasone to pregnant cattle, particularly those in the third trimester, may induce parturition - essentially aborting this pregnancy?
Would you have been unhappy if you'd followed their advice, and your cow had aborted, retained her placenta, developed metritis? Would they have been willing to reimburse you for such a loss that might have occurred as a result of you following their suggestion?
My recommendation... contact and discuss this with your veterinarian...though, from what you've told us...I see no reason for this animal to have been treated with Excede or Sustain III...two antimicrobial agents, neither of which would be indicated for, or have any positive effect on a stifle injury... unless there was an infection somewhere that you've not disclosed to us.