Decided after a couple of weeks that it wasn't for them and resigned. About 5 years later they received a call up letter to go to the gulf. Apparently no matter how long she served she was down as a reserve for a minimum of ten years from resignation.
Are you certain? Sounds totally implausible to me. UOTC can not be mobilised, period, as they have no wartime/operational role. TA soldiers still at university (you can join TA instead of the OTC while at university) can not be mobilised until they graduate/leave.
The solution for Walter Wannabe is to get himself onto a university course that commences before his unit returns from Afg. He'll probably get discharged, at worst he'll be able to defer until he graduates.