Our DS is 5 and its a really difficult age for parties.
When we had his party about 50% of parents left, but they either told us, or made sure that their DC was being looked out for by another parent - much more appropriate than lumbering the host parents who have more than enough to do in ensuring the party goes successfully.
Your DH should not have planned to arrive on time, he should have planned to arrive 10 minutes early at the least. An hour and a half is a very long time if a child is less than happy. Secondly he should have let another parent know that he wouldn't be staying, rather than just the party hosts.
TBH I don't blame the party parents, at our party one father tried to leave his daughter who was crying and obviously upset because he needed to go to the tip
. I caught him as he was sneaking out and presented him with his daughter, he left 5 mins later when she had calmed down and did not reappear at the end of the party, when not unnaturally she became upset again. Luckily she was friendly with another child that was there and the mother was able to look after her until the father appeared.
I did remember about this girl and had the other mum not been there I would have stayed with her until her dad arrived however in the general excitement of co-ordinating the party, keeping an eye on 26 children and making sure DS had a good time at his own birthday party I would not have seen it as my fault if we didn't clock the departure arrangements for each and every single child.
Remember your DH will be presenting this from his point of view. I bet he was more than 5 mins late and I bet he slunk off from the party without pressing his mobile number on to host parent and at least one other parent.
Ok venue was somewhat at fault as they should have very clear sign out policies, but child may have got through with another parent or friend.
I wouldn't be calling the birthday party child's mother, I would just make sure that either you or your DH ( if you trust him) stay with your child at the next party or make darn sure you are back on time to collect.