This might sound a bit daft but here goes. We've had trouble with one of our cats attacking his best friend any time he's had a short stay at the vet or been to be groomed (he has long hair) what we've found is that we give Max a good brush then use the same brush on Henry, after a couple of times it calms Henry down and they are back to being best friends (next time Max goes anywhere Henry is going too) So I think you could try grooming your partner's cat and then use the same brush for your shy cat, though I agree with a previous poster that it might have been a better plan to introduce her gradually into your household