Zimbabwe cannot get visas anywhere except by returning to their home country unfortunately now. Is there a reason they did not get re-entry permits? Unfortunately the entire non B falls away upon exit and far as I know if u leave the country without a re-entry. We have had a Zimbabwean teacher recently and only Non B route is returning to Zim.
Yes the border is very strict now and they are asking everyone for the cash proof of funds. We have teachers crossing all the time for their Non Bs and all have been checked including European passport holders, South Africans and quite a few others. At the moment it seems the American passports dont get asked but this is not a guarantee