You usually find its not possible to split them- for one thing, digging them up with intact roots is usually impossible, and even if you do, you'll probably find all the growth comes off a major root beneath the soil. Get your friend to bend down some growths and anchor them in pots or into the soil - these usually will root where they touch the ground, so when it does root, it can be separated from the mother plant and grown on separately.

I see... What about pruning it really hard after fruiting this year as a way of shortening the size of it width ways?

Rather than one massive prune, it would be better to spread it over the course of three years (which is less of a shock to the plant). I suggest pruning one side the first year, the other side the second year, and some of the very old wood in the centre in the third year.
I would also feed it in spring to help rejuvenate it as well.

Andrew, thanks for replying back. Yes, I'll do that for my friend after harvest time.

Cuttings root incredibly easily. Take 4-6 cuttings just below a bud, break off the bud with your thumbnail & stick it in a pot of compost. Chances are that all 6 will root & be in leaf this Spring. Pencil thick hardwood stems

Cuttings will also root if put in the ground. Make a slit trench with a little sand or grit in the bottom. I used this method when I inherited some old bushes when moving to this garden. The cuttings were taken in September and had rooted by the following spring

The usual time for pruning blackcurrants is immediately after the last fruit has been picked, taking out the oldest wood to the base each time. You can use the trimmed prunings for your cuttings
Removing the weeds from around it would help stronger new wood to grow, as would a good mulch too.