favor作为名词,解释为帮助时,有以下几个常用词组
1.do somebody a favour (=do something for someone)
2.do sth.as a favour (to somebody) (=because you want to be kind,not because you have to)
3.ask somebody a favour/ask a favour of somebody (=ask someone to do something for you)
一般不说do a favor to/for sb.,而是说do sth.as a favor to sb.或do sth for sb.
如果非要用的话,根据英英词典的内容,for 和to应该都是可以的（根据第二条的内容,我更倾向于和to搭配.）