Guinea pigs can suffer from mites, lice, fungal and a whole lot of conditions linked to itchy skin without his/her cage mate showing any symptoms. Although that's not to say his cage mate may come down with problems too if left, if not dealt with swiftly and diagnoises correctly.
The critical thing is that you nip it in the bud and get a firm vet diagnoises and the correct treatment before it gets any worse.
Bathing in salt water will not help at all.
Please take both your Piggies to the vet so they can be properly checked over.