Hello and welcome to the forum.
Firstly I think you need to speak with your parents as ultimately the guinea pig will be their main responsibility and the cost of paying for the guinea pig especially in terms of vet bills will be down to them.
To be honest, I think it is unfair of your mums friend to put you in this position BUT if you truly want a guinea pig and your parents are willing to pay for hi and his vets bills then you could take him provided you can offer him a loving home.
I should point out that vets bills can run in to the hundreds. The most I have ever spent on one guinea pig was £600 (around $900). It is also important to make sure you have access to a vet who specialises in guinea pigs which I know can be difficult in the you.S depending on where you live.
I don't mean to put you off regarding the vets bills but it is something to bare in mind as some people underestimate the cost if their guinea pig were to be poorly, need surgery etc.
I spend around £50 plus ($75) a month on my piggies and that includes dry food, fresh vegetables that they get twice a day and hay but I have 3 guinea pigs.
You may also find the threads in this section helpful. May be you could read it with your parents
https://www.theguineapigforum.co.uk/forums/new-and-wannabe-guinea-pig-owners-corner.52/
But if you can provide all of these things and your parents are happy, then go for it. Guinea pigs make wonderful pets.