First of all Violet, let me congratulate you on your purchase.
Highlands are so easy to keep, very placid and will raise a good calf on an average diet.
They actually have a digestive system which is 30% more efficient than that of the more improved breeds, and as a consequence they do not do any better if you try and force them.
So - you have a healthy but thin cow with a 5 month old calf. If it were me, I would give her good quality hay ad lib, a high energy feed block, and some roots. If you don't have access to roots, then a small amount of concentrates - say about 2lb a day. But to give a lot of concentrates really is a waste of time, as the one thing highlands excel at is their efficient conversion of fibre.
This probably will increase her milk production, but won't do the calf any harm. I leave my calves with their mother till she calves again and have always found they wean them naturally. But keep an eye on them and if the calf is still feeding at 9 months, then separate them and just feed the cow good hay and minerals for the last 3 months. That will be more than adequate for her, as the unborn calf grows most in the last few months, and you don't want it to grow too big.
You won't be able to increase her body score by much for this pregnancy, but will have to do it gradually by ensuring she has good hay or decent grazing throughout the year.