How should one deal with a brother who shaves his beard, having previously grown one, in terms of loyalty and disavowal for the sake of Allah?

It is permissible for a single person to combine qualities for which he deserves mercy, love, praise, and reward, with qualities for which he deserves hatred, torment, blame, and punishment; having both good deeds and bad deeds. Thus, the truth is glorified, and creation is shown mercy. This is the doctrine of the Sunnis and the Community. So, treat him with kindness, love the good within him, hate the sins within him, and invite him to goodness.