A guitar won't hurt a bass amp in any way. Back in the day, more Fender Bassman amps were bought by guitar players than bassists. The first 'real' amp I gigged with was a Traynor Bass Master. Those are still much sought-after these days because they sound like a ballsy Marshall plexi.

A bass will cause problems for the speakers in a guitar amp if they aren't designed to work at those lower frequencies. A guitar head can happily drive a bass cab. Just be aware that you need about 4 times as much power get similar sound levels at bass frequencies. That's why there are lots of 1000 Watt bass amps, but guitar amps usually top out at 100 or 200 watts.