would it be easy for someone who plays guitar to learn bass guitar?

The biggest difference is that with bass guitar, you won't be playing too many strummed chords.  You will be playing more harmony or counterpoint.

Someone who played guitar would know how to tune a bass and how to fret notes. If they'd learnt guitar properly then they'd understand scales, intervals, chords and arpeggios. All of these things would help.

Actually playing the instrument (especially finger style) is different though and the person needs to develop the ability to come up with and play bass lines. So far as reading music goes music for guitar uses the treble clef but bass uses the bass clef.

Early in his career, Paul McCartney switched from guitar to to bass and is regarded as one of the best.

A lot of the skill would transfer directly to bass guitar, though it would not be exactly the same.  

I would think once you know an instrument it would be easy to learn another

There are two issues- someone who has learned to play music on any instrument has an advantage in learning a second instrument, especially if they learned to read music.

Then you have the. physical habits that the fingers learn in pressing down the strings to create the different notes. I know when I first tried to play a guitar, I thought I could never get my fingers twisted into the configurations they needed to be in! Once you've mastered that, there would be a carry-over of skill.