Hmm.. actually if there's no one shooting digital, either u or ur friend shooting as backup may wanna consider doing a digital take. Ideally try to communicate with all the photogs beforehand if u can. Can get some advice, esp from the main photog. & go for the rehersals if it's a church wedding & u're not familiar with it. It'll be especially helpful to know the proceedings for both the church wedding (things like when the solemnisation is, how it's done, & where the signing is, etc) & the traditional ceremonies taking place at the homes.
One other advantage, esp of digital backup I find is that u can shoot things the main photog cannot also focus on. Since there's 2 backups, u 2 can take turns to look out for other interesting moments, like the parents' looks on their faces, the choir, the relatives, etc.
Extra batt is impt, both for flash, & ur cam if u know it can't last the whole day.