The reality is, is that it depends on the union, it depends on the rep, it depends on the member, it depends on the situation.
If I’ve had someone slate the union (reps aren’t stupid, we know most of the members and their stance) and if it’s for something menial like sickness, then probably not, no. I, personally, would ask for their side of the story and if I was particularly aggrieved at the stance management took (on balance they aren’t a member) then I’d rep them - but on the assurance that I want to see their name on the membership list in five years, not dropping off in five months.
Source: ETA TU Rep, Unison