Word for "abrupt stop"

I'm trying to write the following sentence without the adverb "abruptly"

They were walking to the main office, but stopped abruptly when they saw her.

"halted" seems kind of clunky, I'm hoping there's a better word I'm overlooking..

Edit: I forgot to mention, I'd prefer a verb, but if there's a better way to write this, I'm not picky about how.

Perhaps they froze? From the Oxford Living Dictionaries:

freeze

VERB

3. [no object] Become suddenly motionless or paralysed with fear or shock.

   ‘she froze in horror’

   3.1 Stop moving when ordered.

   ‘she came out with a revolver and told the boys to freeze’

So your sentence would be:

They were walking to the main office, but froze when they saw her.

I'd use "stopped short" in this case.

Definition from Macmillan Dictionary:

to suddenly stop what you are doing or saying
I started to explain but stopped short when I realized Adam was in the room.

Or "stopped dead":

to stop moving very unexpectedly

So, in your example, that would be:

They were walking to the main office, but stopped dead when they saw her.

...which, to me, would indicate that they were surprised to see her there, which seems to be what you're looking for.

Two options for adverbs are "stopped immediately" or "immediately stopped"