Yes, as far as I know, you can marry her in the United Kingdom and will allow you to stay in Europe.
Yes he will be allowed to stay in Europe. In Poland to be more specific about it. He could apply for an EEA family permit in the UK, provided that he hasn't been in the UK for more than a year, but first his wife will need to have an EC Residence Permit and she must be exercising Treaty Rights at all times for his permit to be valid.
I'd recommend the original poster contacts Breytenbachs, they're a law firm specifically aimed at the SA community in the UK. Think their website is breytenbachs.com but it might be.co.uk