So you have a friend's bass and want to hook it up to your amp? The short answer to this is yes, but with a few warnings! You'll only want to do so at low volumes.
The reason to keep the volume low is that it could hurt the speakers in the amp. Note that it's won't hurt the bass. The bass is sending a signal to the amp. The bass doesn't receive a signal and therefore couldn't really be damaged. The amp receives a signal and puts it out over the speakers. If this signal is too much for the speakers to handle, they could be blown. So technically the only thing at risk is the actual speakers.
Also, bass amps are built to carry lower frequencies and obviously sound better than a guitar amp. On suggestion is to buy a direct box or preamp that as some sort of head phone option.