Two days is not unreasonable and to be honest it depends on the relationship and your husbands contract of employment. If you have a copy, look at what his entitlement is for bereavement and compassionate leave.
If he was the Grandparents main carer or he was exceptionally close then its all mitigating background. Or maybe his parents were unable to deal with requirements i.e. undertakers or the surviving grandparents grief and it fell to him.
There is no statutory right to paid bereavement leave for grandparents, but there is a statutory right to reasonable unpaid time off for dependants (people you care for are dependents) if the death creates an immediate practical crisis (e.g., arranging care, dealing with urgent matters). Two days is absolutely within the range of “reasonable” to me but he may not get paid for them and that maybe what they wish to speak to him about? Whether its paid or unpaid can be at the discretion of the Manager so the manager may need more info. But as your husband is on a zero hours contract, I personally think the chat will be to point out that he will not get paid.
This is a difficult point because whilst your husband has a zero hrs contract, if he has been working full time for a long time, he could be deemed as having more rights than a zero hrs contract.
We can't plan for such events and the truth is that it sometimes "inconveniences" others but the alternative is having a person at work, who is not fit to be there.
My stepson had 3mths off work when his mum died, we all react differently.
If they phoned in, explained the situation, and followed the normal absence reporting procedure, then disciplinary action would be unreasonable.
Misconduct only arises if someone fails to notify, fails to follow procedure, disappears without contact so if he contacted his boss as per the contract on the Friday and Monday then he should be OK.
But having had the two days off, I wouldn't expect to get a paid leave day for the funeral.