Beer is a kind of alcohol (wine), it is unlawful to drink. You should convince your friend to abstain from drinking and leave this habit. However, while performing salah, he should be sure that there is no effect of the beer. He should perform salah after cleaning the mouth, as the Quran prohibited performing salah in the state of intoxication. The verse was revealed when the wine was not yet prohibited which was later declared haram absolutely.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband