Guinea pigs react very differently to death; the same as humans. It is often not what you expect. Thankfully only a small minority of piggies pine acutely, which is very upsetting to watch. Soldiering on is not at all uncommon.
Please be aware that while guinea pigs are seemingly unaffected they do not require companionship in the longer run.
I am currently looking after a nearly 9 year old lady who was seemingly unaffected by the loss of the last of her long term companions, but over a few weeks became very clingy/attention seeking when in human company and withdrawn/bored as soon as she was left on her own. That was the reason why she came here to live out her life; she's been here for 14 months now. Her loving previous owner and I are both hoping that she is going to see her 9th birthday in May!