If I travel to a destination in Europe that is outside the UK (my home base), I take 100 Euro in cash. There's no need for more because ATM's are available.
If I go outside the EEA, like to the US or Africa, I take 200 in Sterling and the equivalent of 100 dollars in local currency (like Canadian dollars for example). My rationale is that if more cash is needed, then something extraordinary has come up where there's a problem anyway.
The exception being Russia. Although you are supposed to be purely Rubble in everything you do, you only need about 3,000 Rubble with the rest in Euro.
What Mark didn't mention in his answer (+1 from me by the way) is for the generic case where you travel a lot. Then it can be better to keep denominated bank accounts. They can help prevent your getting whipsawed in both directions. I keep a Dollar account along with my Sterling account just for that purpose. Talk to your bank if you think that a denominated account can be helpful for you. These types of accounts are available at most of the major banks in the US.
Adding from commentary
If you are going to be driving and the country levies on-the-spot
speeding (or whatever) penalties, make the cash amount in local
currency enough so that you will be able to pay there & then. Or
expect to have your car impounded until you pay in some countries.
Update (5 April 2015)
You have updated your question to now ask "What types of things is he likely to need cash for?"
In the Netherlands (and most of the EEA in general), he would likely need cash for...
- News agents
- Small grocery stores
- Traffic penalties
- Older style taxis and gypsy cabs
- Open air market vendors
- Charity donations (including cathedral collections)
- Illicit items (out of scope here)
The most consequential of these are penalties which are actively intervened by the police (as opposed to violations caught on traffic cameras). Penalties can vary, but in all except the most egregious cases are less than 50 to 70 Euro...
Foreign offenders will be fined the penalty payable in the country of
the offence, but fines vary across Europe, with Germany levying €10
for a minor speed offence while France fines offenders €68 (or €45 if
In the Netherlands, speeding penalties are calculated with a mathematics formula, but it would be extraordinary to see a penalty exceeding 35 Euro if you are clocked within 6 km/h of the limit.
Violations that are caught on camera are dealt with by a penalty notice sent to the registered owner. For visitors it means the rental agency will pay the fine and debit your credit card for the amount plus their handling fee.