Can I safely use my Macbook battery on another MacBook?
Yes, however there are two versions of the MacBook battery, the Energy Star and the non-Energy Star that are incompatible with each other. So as long the pins look the same in his laptop you should be fine

Check out this post on super user for more details on the different pins of the Energy Star and non-Enery Star connections for the battery.

https://superuser.com/questions/186817/my-macbook-is-a-battery-denier