This isn't just grossly unfair, but utterly insane and may be even illegal. It is the post that is made redundant, not the individual who occupies it. What you propose is a perfectly sensible solution which defers the decision and buys time in case other things develop, such as others leaving. In fact, your colleague's maternity leave alleviates the situation for the year, which means that there is no need for a redundancy because your department will not be overstaffed.. This should be referred immediately to higher authority within the union, i.e. local secretary or county secretary and to regional offices for proper advice.