Shengen Accord allows people within their member states to travel freely
UK has its own procedures
Most new member states are still in the process of implementing the Shengen Accord to which they agreed to prior to joining the EU.
Please check their relevant visa sections of embassies...as for these countries the legislation prevalent before the entry to the EU most probably still prevails.
Many of these countries have traditional cultural/economical links to the USSR.
These links were renewed after the disintegeration of the USSR by what is called bi-lateral agreements.
For example, a women from the FSU of Ukranian Nationality may enter Hungary with less problems than a women from the FSU who is of Tajikistan Nationailuty.
Obviously the particular situation is continuosly being updated in view of these new EU member states reaching the adminsitrative/logistical and financial set-up that is pre-requisite of that member state being ready to implement the Shengen Accord.
In practice however the UK visa is more difficult to obtain than a Shengen visa and in most cases the Shengen visa is more stringent than bilateral agreements. But there are many many (even for EU citizens) exceptions - so be warned!
Anyway the girl should make all the necessary paperwork at her end before leaving to the UK otherwise she probably will only get a short few day visit to visit France or Belgium (say).