can you play a bass on a guitar amp?

You can play anything through an amp. The amplifier is just a means of increasing the inherent signal. The speakers are a bit more of a worry though. It is perfectly alright to play a bass through a Mesa Dual Rec and run it through an Ampeg cab. The only problem is that you will be underpowering the cabinet. Guitar heads (read: amplifiers) are usually only rated between 60-150W at 4Ohms, most bass cabinets can handle upwards of 350W at 8Ohms. The reason for this is that bass frequencies require more power to push the speaker for the same amount of volume as a guitar. A better option for the guitarist would be to just switch with your bassist for the song.