Fibreplex is NOT a recovery/syringe feed. It will not stop weight loss, it doesn’t replace lost hay fibre intake and it won’t keep him stable.
Fibreplex is perfectly fine to give to guinea pigs (many of us use it if you have a piggy with an upset tummy) but it’s a probiotic which helps replace gut bacteria and settle an upset tummy (when they’ve got soft poops for example). You give it a couple of times a day in a very small amount. It is not a food nor a medication.
You MUST give a proper recovery feed such as oxbow critical care or even mushed pellets as the emergency measure. He needs this more urgently than he needs the Fibreplex.
150g is a concerning amount of weight to have lost (particularly if he is small anyway). 50g of loss is the point at which you have to step in and he is obviously much beyond that now - he definitely isn’t eating properly.
First Aid: Immediate Care Measures and Non-medication Products
It’s urgent that you get the scales (so you can weigh him every day yourself) and the recovery feed.
Feed him mushed pellets off a spoon in the meantime.
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
I assume they checked the lump was an abscess or a cyst (you said they did tests so I’m guessing they did a needle aspiration?)
A growing lump should usually be removed before it gets too big and can impede them.
Did they say anything about his bottom being wet?
Is he living inside now?
Bumblefoot (or even just sore feet) can be caused by poor circulation, not moving enough and spending too long in one place, if his bottom is wet then he may be getting urine scald on his feet (hence the advice to use F10 cream).
If he is on disposable bedding such as shavings, then you need to make sure you remove wet areas every day - if he is getting sore feet then it needs to be removed several times a day to ensure he isn’t exposed to wet patches.