Hi and welcome
Where are you located? Could you please add your country, state/province or UK county to your details, so that we can help you better finding a proper solution. We have members from all over the world. Click on your username on the top bar, then go to personal details and scroll down to location.
It is very likely that your girl is already pregnant if she has been with boars for any length of time. Was she on her own in the shop and correctly sexed?
Boars start making babies from 3-5 weeks onwards and sows have their first season between 4-6 weeks of age. Unfortunately, there is no menopause. Sows come into season again within hours of giving birth - they are designed to be nonstop breeding machines to make up for the longest pregnancies with the smallest number of babies of all rodents.
Please separate the two asap. How long have they been together? if your little girl has been near her season, she would have come into season spontaneously.
If you want to take her back, please let the shop know that she has been introduced to a boar and that there is a risk of her being pregnant; you do not want the next owner have a nasty surprise and mum/pups suffering from lack of adequate care.
Otherwise, you have to have your boy neutered, so he can live safely with your girl after a 6 weeks post-op wait to make sure that he 100% safe. He is thankfully coming up to the age when he is old enough for the snip. However, you want access to a good vet to cut down on the risk of post-op complications and you have to factor in that in some countries operation cost are very high; hundreds of dollars, in fact; so you need to do your research!
If want to introduce a new male guinea pig to your boy, please follow our tips for introductions (guides are at the top of this section) to make sure that they have a chance to get on when they meet on neutral ground. If you just drop a guinea pig into a cage, it will be seen as an intruder and there is a major risk of fights or a bonding fail! The key to any happy guinea pig is mutual liking and character compatibility.
UK:
Guinea Pig Rescue Centre Locator
Other countries:
Guinea Lynx :: Rescue Organizations
https://www.theguineapigforum.co.uk/threads/pregnancy-guide.109375/