Most BBs bought at the MTN office are factory unblocked. I doubt MTN locks BBs (at least not mine). If its the case in yhur friend's then she's good to go. Ask her wherr she bought the BB.

Some issues have to b addressed.
First, wherr was the BB bought?
Also, what model of BB does she use?
Was it factory unlocked or locked to any network?
Coz,
Sims work better on factory unlocked BBs than previously locked ones.
Also, some networks don't work on certain BBs.
Its two sided. Phone and Network(Sim).

Hi Siena  - another idea to check if her phone is unlocked: get her to try another local SIM i.e. she could try a SIM from another network either Etisalat, Airtel or Glo.  If she tries it and the other SIM  works without any error message, I would presume that her handset isn't locked into the MTN network. 

That being the case, technically she shouldn't have a problem buying a UK SIM and using it in her phone (so long as she realises the BB services will cost extra).

Also, if her phone is locked, this thread may help, or  Lexusgs430 might be able to help.

Glad it's working for her with no trouble. Hope she enjoys her stay.