It sounds from the two descriptions, that Cambodia is easier than Vietnam.
I am not criticising Cambodia in this next part. I missed visiting there last time, because I was too sleepy and ended up staying in Thailand.
However I know that Cambodia is (at least was a couple of years ago) much less safe than Thailand.
(no information on Vietnam in that regard)
I would say she should probably stay in Thailand. In four days she won’t have anywhere near enough time to even scratch the surface there.