Otherwise known as shaddocks, pummelos, pommelos, or lusho fruit, Pomelos are a crisp citrus fruit that is native to south and South East Asia.
It varies from green to yellow colour when it is ripened and it has sweet flesh. It is the largest citrus fruit you can get measuring between 15-25 cm in diameter.
It tastes much like a sweet grapefruit but without the bitterness.

image wikpedia
So if we can eat them, can guinea pigs eat pomelos?
As per usual, lets take a look at their nutritional data, in particular, their calcium, sugar, fat, phosphorus, and acidic content.
Flesh of a pomelo
Nutritional value per 100 g (3.5 oz)
Energyt159 kJ (38 kcal)
Carbohydratest9.62 g
– Dietary fibert1 g
Fatt0.04 g
Proteint0.76 g
Thiamine (vit. B1)t0.034 mg (3%)
Riboflavin (vit. B2)t0.027 mg (2%)
Niacin (vit. B3)t0.22 mg (1%)
Vitamin B6t0.036 mg (3%)
Vitamin Ct61 mg (73%)
Iront0.11 mg (1%)
Magnesiumt6 mg (2%)
Manganeset0.017 mg (1%)
Phosphorust17 mg (2%)
Potassiumt216 mg (5%)
Sodiumt1 mg (0%)
Zinct0.08 mg (1%)
Source USDA Database entry
As you can see they have a hint of phosphorus, fat, and are a little acidic. However they contain a great deal if vitamin c which is terrific for guinea pigs.
So guinea pigs can eat pomelos one to two times a week in small amounts. But not more than that due to their acidic nature.
But they are a great addition to a guinea pigs diet.
If you are looking for a good way of peeling a pomelo, check out this article