Jackfruit are fruits native to south East Asia having originated in India.

However they are cultivated all over tropical regions including south east Asia, the Caribbean, and South America.

They are often known as jack tree and jak and Its fruit is the largest tree-borne fruit, reaching as much as 80 pounds (36 kg) in weight, 36 inches (90 cm) in length, and 20 inches (50 cm) in diameter. (wikipedia)

So can guinea pigs eat jackfruit and if somehow much can they eat?

Let’s start by checking out its nutritional data and find out a bit more about it.
As per usual we’re looking at the phosphorus, calcium, oxelate acid, fat and sugar levels.
A good amount of vitamin c is also beneficial.

Jackfruit, raw
Nutritional value per 100 g (3.5 oz)
Energy 397 kJ (95 kcal)
Carbohydrates 23.25 g
– Sugars 19.08 g
– Dietary fiber 1.5 g
Fat 0.64 g
Protein 1.72 g
Vitamin A equiv. 5 μg (1%)
– beta-carotene 61 μg (1%)
– lutein and zeaxanthin 157 μg
Thiamine (vit. B1) 0.105 mg (9%)
Riboflavin (vit. B2) 0.055 mg (5%)
Niacin (vit. B3) 0.92 mg (6%)
Pantothenic acid (B5) 0.235 mg (5%)
Vitamin B6 0.329 mg (25%)
Folate (vit. B9) 24 μg (6%)
Vitamin C 13.7 mg (17%)
Vitamin E 0.34 mg (2%)
Calcium 24 mg (2%)
Iron 0.23 mg (2%)
Magnesium 29 mg (8%)
Manganese 0.043 mg (2%)
Phosphorus 21 mg (3%)
Potassium 448 mg (10%)
Sodium 2 mg (0%)
Zinc 0.13 mg (1%)

As you can see it contains a fractional amount of phosphorus, calcium and fat but does contain a lot of sugar so do take care when feeding it to guinea pigs.
In terms of intake, once a month should suffice but don’t feed it to them too often as guinea pigs cannot take the high sugar intake.