Please dont go to the pets at home vet. They will likely diagnoise wrong and give more wrong advise endangering the piggies. They may also force her to hand the piggies back untill they are "better" at which point the miraculously die. ¬_¬ If you tell us where in the country you are we can recomend a cavvy savvy vet (guinea pigs are considered exotic pets)
Here is some information regarding preganacy and how to deal with it.
If you rfriend feels he/she is unable to deal with a pregnant sow please find a local rescue centre. P@H will likely try to get you to hand over any babies born. Please dont. If your local rescue cant take the babies we may be able to organise a piggy train through the forum to transport them to another rescue near by. The cycle will only start again if they are left with pets at home. Please Please be super sure they are both female. It is not uncommon for them to be miss sexed at the shop.
Please try to sorce some alfalfa hay (this will help pups bones and should be fed to mum untill end of nursing period, pups may eat it too)
and an extra Vit C rich veg than usual to her daily veg intake and DONT feed parsley (it can cause miscariages).
If when she gives birth shes having trouble unwrapping the pups, please do it for her this will not harm them and she wont reject them.
If she seems at all stressed by her female cage mates (if she has any) please move her to her own cage. All males must be removed as she will come into season right after giving birth, so she could get pregnant again if he isnt removed. If he has been neutered he should still be removed as when she comes into season after the birth he will still try to mate her (even though he cant get her pregnant) and this will be stressfull for her while she needs to be concentraiting on the pups. Also be aware that as she is a first time mum its possible she may at first be afraid of her babies. This can sometimes be delt with by placing her in a smaller enclosed space like a cardboard box, with her and only the pups so she is forced to accept them.
Once babies are born please weight them (give them an hour or two, but in the first day). 70g-100g is normal, anything under this but over 50g please post for advice on here, under 50g, and post as urgent in the pregnancy and labour section as those/that pup will likely need extra help. (dont be alarmed if there weight drops in the first day or two. This is normal but should then start to rise once mothers milk comes through)
Male pups will need seperating at 21 days old provided they reach 250g. If you need help sexing dont be afraid to post pictures. You will need the space to seperate any and all males, then divide them into pairs and/or fine a friend for any lone males left over if you plan to keep them long term. If you dont (which everyone will understand) you will still need to remove males at 21 days or they can get mum pregnant again. If this is difficult, or you need to rehome any please contact a local rescue. They will keep the pups untill 6-8weeks so they are sure the pups are thriving, and then the home they go to will be checked for suitability (please be aware guineas sold on free ads are prime reptile food- espcially little ones).
Also make sure there is a hidey in the cage big enough for her to give birth in should she want some privacy.
Perhaps you could encourage her to research what a roan and lethal guinea pig is to see if her piggies are at risk?
Guinea pig gestation is around 72 days.
Please have the number of a cavvy savvy vet to hand at all times incase you need it. (if she bleeds more than a teaspoon during/after labour please rush her to the vet. Also rush her if she appears to have a pup still inside and it has been 20 minutes since the last one was born.
Also, very important, only handle her when absolutely necessary. Handling pregnant piggies can harm the pups. If you desperately need her to be moved *ie for cleaning out or vet trips) Try to get her into an up side down cardboard box with a hole in the side for her to walk in and out. (maybe place a treat in there for her.) then hold your hand over the hole while carrying her in the box to wherever she needs to go.
Sorry, i know thats alot of info for you, if you have any other questions dont hesitate to ask in out pregnancy and labour section.