To answer your questions in somewhat of a reverse order:
"...if we buy kits from Ancestry, would taking out a membership subscription alongside be a complete waste of money and time"
Given what you have said about your family background, I would recommend doing a DNA test with Ancestry but, perhaps, just hold off on paying for a subscription until you get the results back.
Ancestry DNA tests usually include a three month membership for £1.
They currently have a half price offer (up until 11th August) for £49 (£39 + £10 postage)
You can also include a three month worldwide membership for £1.
https://www.ancestry.co.uk/c/offers/bundle
I would suggest that you wait until you get the DNA results back before extending the three month membership.
It may turn out that you have a lot of DNA matches on Ancestry, in which case it would likely be worthwhile taking out a membership, or it may be that there aren't many and it might not be so worthwhile.
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"He knows nothing about his late mother's side of the family"
If his father, and/or your parents, are still alive then can I strongly urge you to see if your parents will do a DNA test.
The reason for saying this is that they are a generation earlier than you and so they have more DNA from earlier generations. This will help a lot in finding relatives.
Just as an example, myself and both my parents have done DNA tests on Ancestry.
My parents are in their late 80s and I am 60.
I have 24,132 DNA matches on Ancestry, these are split roughly equally between each of my parents. In contrast, my dad has 23,193 matches and my mum has 27,325 matches.
So, by going back one generation I have gone from 24,132 DNA matches to 50,518 (23,193 + 27,325) matches.
Even if I only had one parent still alive I would still have gained a lot of extra DNA matches
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"...as her father was an immigrant from S.Africa"
There are sites other than Ancestry. One in particular is MyHeritage.
MyHeritage allows you to upload a DNA test that you have done on Ancestry for free so that you don't have to do another DNA test.
The reason that I mention MyHeritage is that I found quite a few DNA matches there that are not on Ancestry. In particular, I came across a number of DNA matches from South Africa.
What I would suggest is that once you get the DNA tests back that you also upload the results to MyHeritage as that may well prove useful.