spend 用法是:
(sb) spend time(in)doing sth
被动形式就是 time is spent (in) doing sth
所以你的例句可以变成主动形式:
He spent his summer holidays in the countryside,(in) helping the farmers with their work.
如果有“in” 那 helping the farmers with their work作动名词,作in宾语。
如果无“in” 那 helping the farmers with their work可以看作现在分词,作伴随状语。