Only someone who had served in the past would have any reserve service obligations. The serviceman concerned must therefore have, once, signed up to fight anyone HMG asked him to. Unless his beliefs are a recent change of heart, he shouldn't have joined in the first place.
I'm sure that, even after PVR, no-one has to spend more than 10 years in the Reserves, so the subject must have either fought in GWI, or joined since - either way, being sent to fight a predominantly Muslim nation shouldn't have come as a surprise.
Are we really that short of manpower that retired reservists are being called up alongside the volunteers?