The part of the rhubarb plant that people eat is the leaf stalks.
Things to know about rhubarb when it comes to guinea pigs:
- The green leaves are very high in oxalic acid, making them inedible for guinea pigs (and people)
- People often cook rhubarb stalks to remove smaller traces of toxins in them
- Rhubarb leaves are so high in some poisonous compounds that humans have been known to have suffered from poisoning after eating them
With all this in mind: rhubarb is not safe for guinea pigs to eat.
Rhubarb is one of those plants that should never be given to guinea pigs.
If you have rhubarb growing in the garden for yourself: do not allow guinea pigs (or any other pets) to access anywhere that the plant is growing.