That’s 57g in a week - That amount of weight loss requires you to go on alert and switch to daily weight checks so you can more closely monitor hay intake - also be prepared to start syringe feeding if the next daily weight check is also down and see a vet.
They are losing weight because their hay intake (80% of their daily food intake) has reduced (even if they are still eating some) (veg is only 15% of daily food intake, pellets 5% - so still eating either of those things, if their hay intake has reduced, will still mean they cannot maintain their weight), and you will need to replace their hay intake with syringe feeding. The reason behind the reduction in hay intake will require vet investigation if it continues
Even if they eat the pea flakes, it won’t make any difference as it just an occasional treat.
We can continue to offer help, support and advice, but we do require more background information -
How old are they?
How long have you had them?
Do they live inside or outside?
You mention the weight of one of the piggies, what is happening with the other piggy’s weight?
Please do read the guides below
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Weight - Monitoring and Management