Was he informed over the phone, text or email?
How long has this particular contact schedule been in place?
Why cant he have them over night - most men would rip their own arm off in this situation?
Step 1. HE(not you) should write a letter to the ex explaining its not in the childs interests they do not see their father and why its important BOTH parents should play a part in their lifes etc, etc. Ask that the contact be resumed as soon as poss - give a 10 working days to respond
Step 2. If she doesn't play ball - are there are family members who can act as an intermediary?
Step 3. Suggest mediation and contact a mediator(prices vary). They will contact the ex on your behalf and hopefully she will come in to see them.
Step 4. You can guess the rest...
HE needs to be conciliatory. You need to avoid speaking with the ex and allow him to do all the communication.
he could look up his nearest FNF(Families Need Fathers) branch and go along to a meeting - he will get sensible and realistic advice/guidance from them.
Don't panic, keep him calm.